RX7 with the Toyota 2JZ L6 engine, which (although heavy) is fairly short compared to later and larger L6.
I doubt an older L6 will be easier (longer and heavier except for Ford 250), and I would bet against an L8 unless the bellhousing is in your lap.
For size, the very low deck height of the Ford 5.0 etc. makes lots of vertical room, the Buick/Rover V8 is shorter but wider, an LSX is lighter.
The smallest package is a modern 3.5 etc. V6 DOHC from a RWD (FWD are cross-axis) Nissan, Lexus, Ford, Chevy, almost 300 hp stock.