You mean 345 30's, right. 345/70's are 37" tall.
That's donk territory. I'm guessing you have a Dominator. 3 circuit probably. Any idea on cam specs? I'm sure the specs look crazy (min 740 lift and 270 deg duration). But on a 598, the numbers are actually nominal. Does the dyno sheet show power levels below 4000 rpm?
Options are TH400 and 4L80E. By the way, I think Shafiroff had DSE build him a 69 Camaro. So I highly recommend that you pick his brain on the subject. If you get that chance, ask about available torque between 2500 and 4000 rpm. I think you will be spending a lot of time in that rpm range, so finding out power levels between 2500 and 4000 (even if they are guesstimates) is key.