gt40p heads have low compression

Earlier this year I blew a head gasket on one side. I replaced the heads with gt40p head that I pulled from a junkyard explorer. The boat starts and runs but has noticeably less power. Before pulling the heads with the blown head gasket, all cylinders were showing compression numbers around 135psi, except for the two where the gasket had gone.

I checked the compression today and with the gt40p heads I am at 90psi on every cylinder. Is this where I am losing my power? Boat is a 1983 sn2001.

Back in June you said it was running great with the head replacement

So is this something that just happened recently or has your definition of great changed in the last few months?

Sounds like you probably didn't check the compression back in June when you put the heads on?

Push rods not correct?

Sounds like the next step would be to do a leakdown test to find out what is not sealing - rings vs. valves vs gasket

Do all parts work on the 351W stock heads when changing to GT40P heads?
Same Pushrods? Same Rocker Arms? Might be a compatibility issue not opening the valves correctly. When I swapped my 78 351W stock heads for a set of ported and polished 289 Hi Po heads 25 years ago, I did have to get custom push rods to keep the proper geometry.
It is not likely that all 8 cylinders have bad wear on the valves causing all 8 to have similar bad compression.
I know many on here have swapped to GT40P heads and someone will chip in this answer.

I did not have the heads rebuilt, as a poor excuse, I was in a rush because I needed the boat as support for a Boy Scout canoe trip. The boat starts and seems to run fine. I did not do a compression test when I put the heads on earlier and only really noticed the "lack" of power when I loaded it up with tons of camping gear. To be honest, this is the only inboard I have ever driven and I don't even know what I should be expecting.

I have to amend my above post as well, I have 90 psi in most of the cylinders, the two nearest the flywheel are reading 120psi. I used the push rods from the original heads and I have not done a leak down test. I believe I used the rockers from the gt40p heads.

I kind of feel like a goon, I was so excited the boat was running after the head swap but it doesn't feel right. I know I say that without anymore experience than the 10 times I used the boat prior to the head swap.. I appreciate all the help from the forum and I love the boat.

Consider one very quick and dirty test,
comp test one jug thats low, loosten the rockers one turn. Retest. If it goes up alot, pushrods are bit too long.

You testing with the throttle open, yes?

Did you drill out the bolt holes yourself?

I read an old post you had about swapping gt40 heads. I seem to have the exact same problem after swapping g40 heads onto a 1995 bronco....low compression about 90psi on most cylinders except the back ones have 120psi. Just wondering what you figured out on yours before I start tearing things apart. Thanks, Erik