I use PST oly in all my builds, but I prefer the firmness improvement over stock. I have seen complaints that they cause rattles through the rest of the car because of their ability to trasmit force. I've also heard of squeaking issues, but I've never had those with PST poly. So, IMO, rubber vs poly is a personal choice of quiet and comfort over firmness and deflection. For a street car, either will work out fine.
Stepping up the t-bars is also a nice step up in improving the cars road feel, responsiveness, and performance. Again, somewhat of a personl choice of how firm, but a .96 in a small block car with all steel panels and/or air condition would be a very nice street set up. Combined with the 6 leaf XHD rear springs, off the shelf gas charged shocks, and a modern alignmnet and suddenly your car will feel almost modern in its road manners. The slightly bigger torsion bars wll also help with front end bottoming situations that 35+ year old t-bars may have. Burder is right, you can turn them up a bit if your ride height is too low, but that will not improve their ability to resist slamming in to the bump stops.
Speaking of which, the factory ride height is measured this way; from the floor to the bottom of the lower ball joint is measurement#1. From the floor to the bottom of the t-bar socket is measurement #2. Measurement #2 should be .75 higher than measurement #1. If you adjust the car to this height, I'm almost willing to bet it will be nose high and you won't like the look. Once everything is back together, set it where you like it and tell the alignment tech to leave the ride height where it is.
While your in for alignmnet, do not use the factory specs unless your using reproduction original tires. Those specs are for skinny, hard bias ply tires. Radials will benefit from a different set up. Ask them to provide as much caster as possible, up to 4 degrees. Get 0 to -.5 degrees negative camber. Finally, 1/16 toe in.
