The LLVs the post office got were based on a 1989 Chevy S10 chassis.
Everything in them directly related to the operation of the vehicle drivetrain is Chevy.
I believe the Urban Delivery Vehicle Grumman sold were made the same way.
But with the full frame of the S10 you could put any drivetrain in it you wanted.
We at the shop are waiting for the life line to run out (10 more years) then we are all going to buy one and build it to our own specs.
One guy has a LT1 drivetrain from his wrecked Vette he says he will put in it to make the fastest delivery van in the east.
Not much info but I hope it helps,
S.