Wakeybake, I have a 1975 CHB 34', but sedan version with single 120hp Lehman. I love the boat, and the engine's reliability and ease of service, and if genuine, 1600 hrs is nothing on one of these if well maintained/serviced. I would think twin 80 hp engines would definitely make it more manoeuvrable and a bit faster in a hurry if fuel burn was not an issue, but hardly any more than a single at hull speed, ie probably about 2.5 gph, as opposed to my 2 gph.
However, the issues to look out for in a CHB of that vintage (experience talking here), are leaking teak decks with water damage inside, or dripping onto and rusting out the tops of old mild steel fuel tanks, also wet core in deck, and rot in the superstructure because that will be timber, possibly glassed over, as that old they were not fibreglass up top, even tho the hulls were.* Might have osmosis also, although one need not be too concerned about small blisters that have been there a long time without change in size.* That'll do I think....