Saw you post regarding OMC exhaust manifolds. You didn't state whether they were the original OMC manifolds or built by Barr Marine or some other manufacture. I myself have a 302 small block Ford, currently running on a Volvo Penta 280 outdrive, prior to was a OMC electric shift outdrive. I also currently am running Barr Marine manifolds, and use the boat exclusively in saltwater. Yes, I've had some experiences in this subject. Since you seem to be located in Texas, you could be either running the boat in salt or freshwater. I'll just cover saltwater use, since it is quite a bit corrosive than freshwater. This article is pointed towards the Barr Marine manifolds, however, can be somewhat used for other types.
How to tell if they are still good. Hum, need to remove the manifolds from the motor and drain. Remove the risers from the manifolds.
Next, let's inspect the interior of the manifolds, the inner most part where the exhaust gases flow. There should be no HEAVY rusting anywhere. If there is, look for a "line" of rust. Will indicate a crack where the "line" is. Especially in saltwater, there will also be salt deposits where the salt has been solidified by the hot exhaust gases. Do this for both sides. If the interior looks mostly black then let's move on to the risers.
Again inspect the inter most interior of the riser. Follow the instructions in the above paragraph. That will cover the interior portions of both items.
Next, lets move to the water jacket area, this is the area BETWEEN the very outside of the manifold/riser and the inter most interior. Here we are looking for rust, yes there will be rust, but the rust we are looking for is of a "flaking" type. This rust is of large enough size to block the water outlet areas in the risers. If there is a large amount of it present, you can "rod" it out using a coat hanger. This process will take a couple hours, and have a air compressor and air blow gun handy. If the rust is really excessive, like you have a nice pile on the floor around 4 inches in diameter and 1/4" high, time to dig in the pocket book!
Now let's move back to the risers. Inspect them as in the above paragraph. One difference, with the round end toward you, especially with the Barr Marine type, look inside the riser and you will see a stainless steel tube. This tube directs the exit cooling water around the exhaust gas stream, ensuring the water is not "sucked" back into the manifold area. Over the top area, there will be an exit hole, hard to see but it is there. Should be around 5/16" deep and around 3/4 wide. This is where all the cooling water passes. If this hole is blocked, the engine will overheat, and along with that, the hot exhaust will burn the rubber coupling hoses on the exhaust system. Make sure this hole is clear of any matter. On the Barr Marine manifold, look very closely on the bottom area and find a 3/32" hole, it is a pee hole to cool the lower portion of the riser. The area between the stainless steel tubing and the outside of the riser must be clear. Chip that rust away! If you run in saltwater, you will notice a deposit around the inside of the stainless steel tube where it meets to the inside of the riser. This is somewhat normal, and cannot be rectified. Just chip the deposits and continue.
Once you have completed the above, let's proceed to the mating surfaces that mate the riser to the manifold. This area MUST be clean, no dark spots and no rust. Those dark spots show the corrosion effects on cast iron when not subjected to fresh air. Best way to clean them, take them to a machine shop and have them surface ground. These surfaces MUST be FLAT, absolutely critical to the sealing between the riser and manifolds. The gasket between the riser and manifold, use ONLY the manufacturer's gasket. They MUST be soaked in oil prior to installation, do so for around 30 minutes. The oil will make them "swell", torque the riser, this will make them "conform" to both mating surfaces. If all looks good, install the risers on the manifolds, then install the manifold assemblies on the motor.
A hint, you can use automotive exhaust gaskets, however, I have too, but have found much better service with the Barr Marine gaskets. They still cost almost the same. Make sure you torque the manifolds and riser bolts EVENLY. The Barr Marine risers are around $95, while the manifolds can be had for around $193. These are local prices here in Hampton Virginia, you can probably find them just about the same mail order, maybe 20 dollars or so less, but shipping will tack on a pretty penny!
A couple of notes,
DO NOT run the motor without water supplied, this will not only significantly increase wear on the water pump, but will severely limit the service life on the manifold/riser gasket!
If water, especially salt water leaks even a little from the manifold/riser gasket and drips down to an exhaust tube next to the engine head, the cast iron will flake off quickly, leading to another manifold replacement!
If you run in salt water, flush the engine EVERYTIME, yes I'm guilty of not doing so, learned my lesson the hard way...
DO NOT use the stainless block off plates between the manifold and risers UNLESS you have routed a water tube between the manifold and risers. If you plan on putting the boat up for a couple weeks, REMOVE the drain plugs from the manifolds. This will allow the water jacket in the manifolds to dry out. Definitely will decrease corrosion. Use a non-hardening gasket sealer on all thread items, plugs, etc. Use BRASS plugs , and torque firmly but not with much pressure around 10-20 ft. lbs., throw those iron ones away!
If your manifolds are bad, time to start looking at replacing the water-circulating pump too. Another 180 dollars or so. They can be had from $120 to $212, just have to "shop" around. No, the automotive version will NOT fit, I tried it. This pump looks the same but is shorter in height. :-(.
If you have any other questions, please ask!