Chris,
I haven't concidered it, but really like the 55's heater.
It's iconic to the period and I do, too, but can find no way to maintain it in the OE location without needing to raise the roof 2", considering the buckets I've looked at.
The 60/80 heater would fit and allow the headroom requirement or exploring the possibility of relocating the OE heater in between the front buckets.
Running lines external the tub was just a thought after crawling around on the 80 to measure the heater. I dislike the overall design of the 80s hosing for the rear heater, since it can fail....but most at 18-19 years, but my thinking was eliminating the additional radiant heat source in cab, plus eliminating the hoses, allowing for a form fitting flooring.
Strict restos are much easier......I think.