They cost exactly the same to build, one has much higher power outcome. If building motor from scratch, a 377 is $50 more than a 350 and that is a whole lot more power.
The small bores of the 5.0 requires small diameter valves which limits power. No you can't run 350 heads on a 305
FWIW. my RV is just over 1 year old, just a few weeks past the warranty period and the plastic fittings on the PEX are leaking
if you are redoing the plumbing. start new. I would probably spring for a new $70 fresh water tank
additionally with you 35+ year old boat. check the structure
Cranking psi is listed at 150 psi in the factory manual. Your compression numbers have a motor that will run, barely, however be down on power. I would pull the motor and plan on replacing. A 5.7 will bolt right in if you choose to upgrade
Start with the hose fitting on the drive that connects the hose to the transom shield which feeds the raw water pump
That fitting is part of the annual PM where you pull the drive and inspect things
I have an old 4.3 dizzy body with the gears ground off as my priming tool. if you pre-soak the push-rods in a skinny container of oil, it fills them up, less air to displace.
Block and heads are same GM parts. All others are specific Mercruiser or Volvo.
Make sure you are getting same generation of 4.3. there are 7 generations of 4.3s, and while you can cross generation lines in some cases, it's more work
Isolate and block the line from the raw water pump and the lines to the manifolds. Drain the block and heads of water, and pressurize to 15psi. It should hold pressure forever. If you loose pressure, listen for the hissing. That is your leak
