My everyday boat is a 20ft century boat with a omc outdrive the motor was replaced at somepoint with a 188hp mercruiser 302. Boat runs drives good but last time out i found the oil milky not sure if head gasket out or what but figured its a good excuse to throw a set of gt40p heads on and a 4 barrel. Was wondering if anyone has ever done the swap? Will the exhaust manifolds clear the spark plugs? Im guessin it'll be a 30-40hp bump, will i need to reprop? If i come across a fairly cheap 86+ 5.0ho with roller cam n forged pistons i might just freshen it up and swap it in. Any cam recommendations? Fairly heavy 20' (assuming stock prop?) Omc outdrive (77) 9.5-1 comp gt40p head and 600cfm 4barrel

Your going to be hard pressed to find current people playing with old ford smallblock marine motors...... however I think Holman Moody was the company with ford marine motor stuff back in the day, and they are still around. Just going to a 4bbl and a better cam should get you that 40 hp with the original heads. Just a thought, you may get more for the same dollars going with a sbc,,,,

there is a GT40 head and a GT40P head. the GT40P has different spark plug angles and I will bet they will not work with the manifolds as they will not work with a stock auto manifold or headers. in fact no one I know of makes a header to fit the P head. if you can find the earlier GT40 head then you should be ok.

Ive been looking into the gt40/gt40p heads for a while now (have a 18' flat bed with beaver tail i haul vehicles with that im putting together a sbf for havnt decided on 351 or 392 stroker yet) the gt40p heads will clear a stock truck manifold and long tube header for trucks and 2-3 shorty headers according to mustang and f150 forums. I know the gt40 heads would work. But i want the revised conbustion chamber of the p head its fairly more efficient then the older head. Im not lookin to build a power house ny any means. (Got the 454 carrera 7m ss for that) i lookin to improve efficiency and if it happens to make more power.... thats ok lol

P.s Id only be changing cams if by chance i find a roller cam 5.0 ho short block and dont rob the forged pistons to build the 351w stroker.

I think the cam change would really help. the 188 is a 2 barrel (I had a pair of these in a boat) and really didnt rev out.... Going to a 4 bbl and better intake will help it breath a lot better, it will want more cam (nothing crazy) to take advantage of the better breathing, especially up top.

OMC 1977 outdrive? Electric shift? If it is and was originally a V8 drive it will have the eight set screw spring hub rather than the standard four set screw hub. I did run a 351W with a four barrel and thru hull exhaust in front of a 1977 OMC electric shift drive. The exhaust is just as important as the added fuel & air flow. The Barr type as well as the original Mercruiser style like many others lack the flow to allow a moderate increase in horsepower. I have seen the roof of the exhaust port on the Ford heads leak on vintage applications. Your exhaust manifolds and risers also prone to failure with age. Perform a leak down test or at least a compression test to be able to scrutinize the cylinder or cylinders effected.
This is the time you may want to really think about spending the money on heads and exhaust (unless the exhaust is new replace it {manifolds & risers} not worth the risk not to) on a vintage engine. You could just put a new 225 HP 4.3L Merc in it and get 30 HP with a full factory warranty. You could get almost 90 HP going to the 357 Alpha 4bbl. Also a marine specific rebuilt Ford small block. I do like the distributor in front.

im pretty sure its not electric shift, it does have electric not hydrolic lift with a motor hammer coupler and gear (hammer couplers broke outdrive is stuck all way dowm atm... yes it's on the top of to do list lol) it does have hydrolic motor mounts for trim n that still works. I was thinking the exhaust was gonna pretty restrictive, another reason why not really tryin to go all that big with power increase. Them heads are dirt cheap and i scrap for a living ive bought whole explorers for $75 at that point pullin heads n scrappin the rest would pay to have the heads freshened up. The boats in fairly good shape (paid $400 for it) carpets nice has new bimini top seat vinyl still soft they have a couple holes in em but usable even with the closed bow its very open can seat 6 easily and the deep hull handles big/rough water with ease. i got it to bridge the gap between my 16' alum fishin boat and the 24' carrerra. And I'm only allowed to go "crazy" on one boat lol. But if head gaskets are leaking (still have to confirm this i left the lil "hood" up on dog house and it had rained pretty good the dipstick end is shaped like a funnel. theres a small chance water got in that way. I just changed oil b4 putting it away last season) sorry im rambling just tryin to explain why im not at all lookin to repower it. Had always planned on goin 4 barrel (would like efi if it was a bit easier to diy a marine app) if heads comeoff i figured no reason not to throw something lil better on

Does anyon know if the marine ford v8s shared the same 4 degree retarded timing chain like the auto/trucks?

that may be, all I can do is tell you what I have been through with them. I tried several sets of headers the they claimed fit and none of them would clear the plugs properly. I called several of the header makers and they all told me no one makes a header to fit. I had two sets of factory manifolds for the standard head and there was no way they would clear the plugs. the only way I made it work was cut the headers up and modify several tubes to clear. with marine manifolds I cant see them working but who knows till you try.