Fitting a longitudinal mount in the back seat is like putting a square peg in a round hole. It can be done... but you'll need to make a bigger hole.
A longitudinal engine built into the back of a regular car would necessitate either moving the rear axle backwards or the front seats forward. It's a poor fit. A transverse engine will sit properly behind the front seats, and you will be able to reach the accessories from the front.
A longitudinal mount will work. But a Beetle box for a V6? Those things can hardly take modified Beetle power (which is why Beetle draggers go for Samba gearboxes). You want something bigger, definitely.