Here's what I can say for sure- for awhile I was running a 32/36 Weber with the dual-into-single adaptor that
www.stoveboltengineco.com sells. While it worked ok for awhile, it wasn't that great and too small for the engine/truck combo.
Although the Holly carb you're considering is bigger, I suspect you'll still have issues with the fuel feed/distribution trying to get through the stock small hole. I suspect it would take the form of super lean #1 and #6 cyl's. Personally, I'd prefer an Offy 4bbl manifold and get the Clifford 2bbl-into-4bbl adaptor if you've just got to have a 2bbl. I just think you'll have better success and driveability. Also, before I got my 4bbl setup, a local import tuner mechanic said I'd get better throttle response and performance with a 500 2bbl based on his experiences with others that had done it. I never did find out for sure.
Fwiw, I have the adaptor I just mentioned and I just took photos the other day to post it up for sale. I'll offer it to you first if you want it.
I always thought it would be really cool to machine the base of the stock manifold to accept a big 2 bbl manifold like what you're referring to. I just never tried it. You might still need an adaptor flange, but would look and probably perform better. Maybe you have some machinist friend that could do this on the cheap. We all know the stock intakes are throwaway parts and it won't hurt if it doesn't work and you have to get another one.
I just noticed you're in FL. A guy on the
www.inliners.org site with the board name Twisted6 (Frank) is in your same town I believe. He's one of the nation's experts on these L6's and hopping them up. He'll know for sure what will and won't work.