AA:

Not only is the engine bay narrow, but so is the transmission tunnel.
If you depart from MB transmissions there is a considerable likelihood of doing floor modification.
The narrowest of US V8s is the pushrod Ford 289/302 series. The AOD transmission that mates with those engines does not require any electronics, but will need a bit of floor bashing to provide clearance at the right front corner of the tunnel.

60 degree V6s are also narrow, but a bit tall.