Rick,
It is my understanding that the Berg 58mm (and not the ultra rare Weber 58mm) are a direct bolt-on to any 48mm manifold without modification. This is from the geneberg.com website:
"These are a Berg casting that we machine to 58mm. The industry standard 48IDA bolt pattern and dimensions are maintained so they fit all 48IDA manifolds without modification or adapters including V8 applications"
By the way, they now make the carb up to 61mm.
Webers are the best!
Alan Sorkey
Shreveport, LA
CSX4083