Though I would post here since limited response on DIY engine section. I am building a motor for a 1986 22' Velocity project I acquired. Boat had a BBC Mercruiser serial number F810042 which I believe is a 7.4 Bravo Gen VI motor vintage '96 or '97 that was cracked. I have a Mercruiser block serial number K189211 that is missing the main joirnals. Questions to you parts guys. Will the main journals from the cracked block work on the other motor if it is line bored? Both are 4 bolt mains. Also, I can use the heads off the cracked block or a set off of a 1990 502/465 HP Motor. The cracked block heads have casing numbers M125 10114156 and the other are C080 14036188. The latter heads also have HI-PERF cast in them and are larger valves with rectangular intake ports. I have an Edelbrock tall intake manifold and a Holly 780 carb. Will the stock Mercruiser exhaust manifolds, risers and 4" hose work OK? I do have a set of Gil Big Boy manifolds and Stainless Marine tailpipes off a 572 blower motor but would have to rework the tailpipes and cut larger holes in the transom for the 4 1/2" pipes to work. What kind of hydralic cam should I use with the roller lifters? Lastly, what piston and compression ratio do I want if I only have access to 89 octane? Any advice or counsel would be much appreciated.

Motors are electric and engines are gas or diesel powered,I hope you are building an engine.SORRYYou really need to talk to a marine engine builder,JC Performance,Sterling Teauge etc to see what will work together and what will not.Good luck and remember motors are electric and engines are gas or diesel.

Right you are! What size batteries would I need for a 500HP Motor? LOL. Thanks for the response and advice. I do have an engine builder I use but was just looking for other opinions as well. Enjoy the last days of this boating season.

First off do not mix parts from different gen engines. Using parts from a gen IV on a gen V or Gen VI is a bad idea IMHO.
Yes, you can use the mains from a different block if you line bore. Again gen IV has a 2 piece rear main seal, gen V and gen VI, one piece.
The core/water holes on a gen IV are vary different from a gen V or VI. There are special head gaskets, but...
9:1 for 89 octane and hydraulic roller lifters for 500HP.
You don't need dry exhaust, but you do need a longer than stock riser.