If you are going through all the work of adding heat, whether it's electric, diesel or coolant based, the best option in my opinion is always coolant.
Least amount of power draw, least chance of fire, and most available on the open market.
The cost, work effort & installation time will be similar with all 3 options.
Most 753's have easily accessed coolant ports and are easy to plumb & find electrical power for a fan.
An off the shelf heater box can be floor, footwell, sidewall mounted, or even back window will work.
Just my opinion of course.