I'd get the thin plate that covers the bowed out secondary flanges on the spreadbore intake (it's cheap) & run the old holley & see how it goes. I think the bowl vent was no longer vented out to the atmosphere due to '68 emission regs & instead used a nipple that ran with a hose to a charcoal canister which might help you ID the year just FYI. As said I think the 750 might be a bit too much even for a lead footed teen but if it's in good shape it would get him up & running if the other one needs rebuilt