Gen V and VI have a one piece rear main seal, and the provisions for a roller cam.
My Gen V did not have a roller cam in it, just a flat tappet hyd, from Indmar Marine, new, in 1996.
Make sure it has a roller cam before putting the roller lifters/dogbones/spider in it.
Otherwise, a 454HO is a decent package for a jet boat, A impeller
Some specs (cut and pasted):
Camshaft: Steel Hydraulic Roller Tappet
Cam Lift: .510” Intake / .540” exhaust
Cam Duration @ .050”: 211 deg. Intake / 230 deg. Exhaust
Lobe Separation: 112 Degree
Cylinder Heads: Iron Rectangular Port.
Combustion Chamber: 118cc
FWIW: Gen V heads will only fit Gen V, But Gen VI and Mark IV are interchangeable with the right head gasket.
Dan'l