454's
 

I would not do anything to them. I would bolt on a ProCharger on each engine and call it a day. You could remove them later if you wanted.

There has been alot of good tips, & ideas on this thread, but I notice that all the suggested upgrades deal with carb set-ups, I would like to do upgrades to my 454 mag, & retain my EFI. I'm looking to be in the 450 hp range, would some of these same suggestions work with the EFI to get me in that range? I'm open to an all out rebuild after this season, I am just trying to stay away from spending 15 to 20k on a complete engine replacement.

If I were you glassdave so for a budget engine I´d search 049 or 781 ovals machine them with big valves and some pocket porting.
Slap an Air-Gap on them and a 800 marine carb.
Plus the mentioned Crane upgrade roller and rockers.
I wonder why people believe rec ports are better on a boat as they will outpull a oval port only at above 7000rpm´s before that they are lagging so much you won´t believe. The Dart ovals are a step up choice to the GM ovals and they are brand new thou...