Boat T-STATS are a lower temp, and are all stainless whereas most cheap auto stats are regular steel. A steel t-stat in a marine environment will NOT last long, and WILL make a mess of things. And don't forget the check balls in the T-stat housing. It may be worth your time and money to just buy a whole new housing with check balls and springs off EBAY.

ok, where are the check balls and how do I test them? I still have the tstat housing off, I need a gasket.

This work thing gets in the way...

I just looked back through some of your posts and you have the log style manifolds. It doesn't appear as though they have the check balls in them. Check ebasicpower out for parts, that is where I bought parts for the last I/O I did.

I had the boat out for the afternoon today. It ran great, I had it full pin for about 10 min and the temp never went above 160. So the water impeller was the fix. So..

WOT is 30 mph and my rpm is 4 grand. So I think that means the prop is right..... :beer:

nope that means that either you aren't making the power you should or your prop it too steep... you should be hitting 4400

I have another prop I can try....

try a timing light too.
what pitch are you running now... probably a 15 would be right... I bet you have a 19 now

fwiw my cruiser runs 17's and tops out at 48 mph and 4600 rpm

The older 350's like his are rated at 4000-4400 I believe as I know a lot of the old timers don't like to spin them faster than 3800-4000 when they prop them. So at 4000 WOT it isn't that bad, but there is room for improvement. Also verify your tachometer is correct before you go much further, but I would think a vessel of that size should do well with a 16 pitch large diameter.