Id run your stock heater, with a voltage drop in line to get 6 v. to heater motor.

https://www.fillingstation.com/detail/4185/Chevrolet_12_VOLTTO6_VOLT_VOLTAGE_DROP.html

Rerouting hoses will be a pain no matter what heater you use, maybe under floor??????

Last edited by JiMerit Boltr#43; 05/21/2020 7:18 PM.
It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed