Pickups do offer a lot of space to work with. For examples, check out the professionally designed conversions of a couple of decades ago - the Solectria S-10 (based on Chevrolet S-10) and Ford's own Ranger EV. They don't use space in the box, but they are pure battery-electric designs, so they don't need to accommodate the fuel tank and exhaust system; however, if you have a look I think you'll find substantial space possibilities under the box, especially without a shaft down the middle and especially if you are willing to relocate the exhaust system.
One challenge is that almost all pickup trucks - and all used pickups that you can get cheaply - use a beam axle in the rear. That means to connect a drive motor to requires either complete replacement of the rear axle (e.g. the Ranger's DeDion setup, or some independent suspension) or a bulky installation which retains a driveshaft. If using a Tesla drive unit, it would make sense to choose a highly compatible suspension, since there's no usable original suspension to work around. I think you can assume that after adding a Tesla drive unit to the back of a pickup, there will not be enough room for a spare tire, unless (like the Honda Ridgeline) you mount the spare very high, over the motor.
For an offbeat alternative, if you can find an old Rabbit pickup or Dodge Rampage that hasn't disintegrated into a pile of rust, they were front-wheel-drive so they already have no rear drivetrain. Their rear axles cannot be driven, but presumably you would swap a complete independent rear suspension from a RWD or AWD vehicle under the back. They are very small as pickups go, so they might not be functional enough to be worthwhile, depending on how you would use the truck.