what size torsion bars should I buy?

This is very subjective.I replaced my 340 .870 bars with nos .890's and the ride
was still oozy.I now have .920's with Bilstien shocks and it has a more modern feel
but you will notice rough roads.I think 1 inch would be unpleasant on our terrible roads.
..with the .920's and Bilstiens you can actually push it a bit and i think the ride is great
on decent roads.If you are blessed with lots of new pavement 1" would probably be nice.