Depends on how much you want to spend. The pickup roof skin has no drip rails but it could be as base. If you have good body work skills it would be doable.
Or try to get a good roof at a junkyard...
Maybe the po changed the heater core when the ac was installed. Is there a part number on the old heater?
Measure the inlet and outlet size too and search by dimension on summit. I came up with this, not an exact match but they could fit.
https://www.summitracing.com/parts/sum-382140...
Back from the garage, yes the pinion is offset. These are the measurement from my 74 C20.
I measured from the inside of the spring perches.
Perches to pumpkin: D: 8.2 P: 12.4
Perches to pinion: D: 20.65 P: 19.5
I learned something today as often in this forum:)
I would not use the undercoating. It hardens over time and starts to crack. Water gets in and creeps under the undercoating. The rust can not be seen and when the undercoating falls off you have serious rust problem. That's my experience with undercoating products.
I use a good primer and 2K...
12° advance is a good starting point. You want to set your total timing which is around 34-36° without vacuum advance.
Search youtube or google, there are many vids for setting the timing.
https://www.motortrend.com/how-to/how-to-set-initial-timing/