You might try www.cliffordperformance.net
or Tom Langdon, www.stoveboltengineco.com
Jegs and Summit both sell Holley 2 and 4 Bbl carbs and Edelbrock 4 Bbl. Two barrel Holleys come in 350 and 500 CFM. Edelbrocks start at 500 CFM (but no 2 Bbls).
www.jegs.com
or www.summitracing.com
I don't think you can squeeze a 4 Bbl onto a stock manifold. You may even have trouble with the aftermarket 2 Bbl types. However, I am completely unfamilar with the early L6 engines. When I converted my stock '68 292 from a Rochester Monojet to a Holley 390 4 Bbl, I had to replace the stock manifolds with an Offy 4 Bbl intake and Clifford headers. I was pleased with the results, but it was not as expensive in '78 as it is now to get the same parts.


Every 25 years I like to rebuild that 292 just for GP