IMHO I would not go for the 2bbl to 4bbl conversion. It sounds like you
want more power. I would upgrade to multiport fuel injection. I helped
my cousin who has a donzi with a 4.3L convert from a carburator to fuel
injection. He got the manifold and stuff from a smashed S10 blazer in a
junk yard. The computer, distributor and wireing harness came from some
place I forget, its been 3 years. We got all the mechanical and
electrical done. Got every thing running in the driveway. Took it to
the river called the computer people hooked my laptop upto the engine
computer they walked me through the programing once we were done he had
more HP, better throttle response and better fuel economy to boot. The
who process took a weekend to install. and about two hours on and off
the phone with tech support to get the computer right.
It still runs great today. The only problem was last year the plastic
hose from the fuel pressure regulator (under the manifold) cracked. A
common problem with the Chevy 4.3L.

Paul