Volvo used to use Holley carbs after the Rochester went out of production, so it might be easier to deal with the throttle cable hook ups, fuel line etc. Take a look at parts diagrams to see what you'd need. If you can get a cast iron spread bore intake, the Holley 4175 is made for that application, it was intended for a Quadrajet replacement. If you go with this carb, no need for an adaptor which would be needed for a square bore carb like all other Holleys and the Edelbrock. If you can't get this one, any 4bbl Holley of the right CFM will work, but you will need the adaptor unless you get a square bore intake (Edelbrock) for marine use.
IMHO the Quadrajet was an excellent carb, engineered for reliable operation on a very wide variety of GM cars, from small blocks to 7.4 and 7.5 liter big blocks, but the last ones were made in 1989 or so, so any "new" ones are remanufactured and of questionable value. I have an original un-molested marine Q Jet on my 4.3 and it's been very good, I can get just about any part I need, but I do have a spare carb set up for this engine in my garage (Holley 4160 that was actually jetted for the OMC/Volvo 4.3).
Pros: you will never be sidelined with the NLA parts BS that both Merc and Volvo put you through on older EFI engines, won't need a high priced pair of electric fuel pumps, will always be able to get parts via Holley and other aftermarket
Cons: you have to learn how to tune a carb, for younger folks this is a lost art, but for us oldsters, well all our cars were like that!
They don't start as easy or run smoothly cold like an EFI engine, you have to know how to start an engine with an auto choke and allow it to warm off before casting off.
For me on an older boat, I'll take a carb ANYDAY over marine EFI with its multiple points of failure wiring, expensive parts and NLA parts.
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