Assuming you want to use the larger 73-76 balljoint there are a litany of option from ebay to Firm Feel. For minimum resistance I would choose an arm with heim joints. If using an arm with poly bushings make sure it is greasable and that you use the correct silicone grease. I have used ebay tubular control arms and grovved the poly bushing in a lathe along with adding grease zerks. Careful with shock clearance on the ebay stuff.

To get maximum downward travel (UCA travel down as body rises) the frame rail upper lip will need to be trimmed where it interferes with the UCA. At that rate your binding is at the ball joints. Using high angle ball joints would be something to try although you would be on a mission of discovery there. If you can physically get more travel there will be an alignment issues to correct, especially in the bump steer measurement.

That said, a Mustang II style strut suspension calls for 2.5 degrees positive caster in a light weight tube car. What are your alignment goals?