my 52 has it under the passenger floor mounted to the frame just as your 51, and my plan is to remove the mount from the frame and bolt it to the passenger firewall. My truck will be a driver as well and for ease, I believe having it in the engine compartment would be best.
Im doing a bit of custom work to the 52 (ie door poppers, no handles on the outside, etc) so having it under the floor mounted to the frame would make it a bit more harder to get to if I had to recharge/jump the battery in case of emergency, though I am adding an emergency locked out latch to be safe. Either way it going in the engine compartment.
