Can anyone recommend a good set of headers for the A100 pickup? Using a standard bolt pattern 360 head on stock 273 mounts with auto trans.

I can't give you a 100% guarantee, but I did look for headers, and from the research I did, B series van headers should work, you might need to trim a little around the floor opening.
Based on that I ordered Hedman Elite series headers for a van, but here's the catch - they're not made anymore, and the lesser models have really thin flanges and tubes. I got them from some place that had one set.
Now, why do I say that I can't guarantee the info?
Because I haven't installed the headers yet!
So, take it with a grain of salt.
I would think that truck headers (2X4) might also work. You're looking for headers that would be similar on both sides, since you don't have to jump over a steering column on the driver's side.

Good luck,
Ran
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I you look at that area, you can see that the headers can't swing out too far from the head or go up (it will run into the insulation on the doghouse). However, you have almost unlimited space below. I have heard of A body headers working.
I'd imagine block hugger headers would work too.
If you are adventureous, they are probably the easiest ones to fabricate out there, due to the doghouse, mounts and starter being the only things to avoid.