The nose is too short for a chev. look for a later model Mercedes Engine such as M110 . They are tough,(hemi combustion chamber) and for the money you will spend changing the 4.10 rear end to gear it up to suit the short stroke GM engine,you could turbo the M110. Another choice would be a m103 single cam six or M104 twin cam six. Both engines are smooth and powerful and will fit without issue.
I am dropping a Rover V8(UK Buick 215) into a W11o at the moment for a customer.

These are small engines but a lot longer than any mercedes engine so it going to have to have a lot of sheet metal mods to fit a radiator up front.
If you had access to one,a OM606 diesel turbo would make for huge power and low running costs...