How bad is this tire/gear combo killing me??

The car was originally set up for a ~25" tall tire. You now have a tire that is 3" taller or ~13.3%. So you've taken the 3.23 rear and effectively made it a 2.8 ratio with the taller tire. If you want it to run similar to the stock set up, you need to install a 3.73 gear.

That's a tall effective ratio set up for a 4 speed car. Especially a 340. Put/try/borrow a shorter tire on it see how it wakes up!