Looking at your numbers I think the evap and compressor are working fine. What are the conditions under which you see 340/55? Is it 2000rpm with a heat soaked car in the sun? Probably reasonable for those conditions.

I'd be looking at condenser performance. You need to dump more heat to bring the pressures down and get better cooling. Given where you live perhaps hacking together a parallel condenser mod like the 123 guys is the way to go.

About the best performance I ever saw from a 124 is discharge air that is a 30F drop from ambient. So it's important to have the 100%'recirc function working, otherwise you will never see cabin temps where you want them to be.

What compressor does the TE use? My TD wagon with the 10p15 always struggled in Texas heat, but the '93 2.8 sedan with a 10p17 and factory 134a was almost uncomfortably cold in the same conditions.