Steve have you thought about losing the 2 speed Powerglide and installing a TH350 in it's place?
It is just about a bolt in swap except for the kickdown cable hook up which you should be able to do using a cable and bracket set up from the later inlines that ran the TH350. This will give you 3 speeds instead of 2 and acceleration from a dead stop will be much better with the lower first gear.
No amount of adapters will ever make any 4 barrel carb work right on the stock manifold. I'd put what I already had into first class operation and tuning and save up to make other changes down the road. I don't like spending money for the same thing twice and you probably don't either so I try not to rush into changing parts and systems just for the fun of it and then when I do a real performance mod end up tossing what I did a few months ago and doing it again to suit the latest changes.
