How much fab or changes if any are needed to go from a 454 to a 500hp blue merc

If you are talking about swapping out your 454 to an hp 500, then its pretty much a drop in. If the 454 is a Gen IV then there are some very minor differences in the bolt size that holds the assessories on the heads but thats about it.

you will need a hd coupler,and a cooler and oil thermostat that comes with the hp500,if you are replacing a black motor.

u will also need to upgrade the exhaust system,or you will get reversion.