OP
The O/D box has terrible ratios for hi-rpm use.The spreads are ridiculous for that.
The ratios are 3.09-1.67-1.0-.71/.73. The spreads are 54%,60% and od. What that means is that during a shift at 6500, the rpm will fall to those percents; so 3510 in first and 3900 into direct (3rd). The little 340 will fall on it's face, to drag itself from those rpms, unless you build it specially for that spread. with 4.30s it will have to pull from 75mph to trap speed. That is a long hard pull,starting at 3900, some 600 to 800 below peak torque. And first gear at 3.09 x 4.30 =13.29, is equivalent to running a 5.00 with a 2.66 low tranny. So at the track you will shift into 2nd,pretty much right out of the gate and have 2 gears to run the qt, outshifting 2nd at 75 mph.Again a long hard pull.
I ripped the MD teeth off one of those gearsets at about 400 to 440 ftlbs, and I wasn't even using all of them.
And I shredded two OD gears.One with a teener.
The BB has so much inherent torque, you wouldn't have to shift at 6500. But you already know it's the wrong tranny for the high reving 340, so IMHO; fugetabowd that O/D box, for your stated useage..
Now, if you would consider a 360 with a GV, I can tell you that is really really sweet with a cam of 220 to 230 @050, and up to 3.91s!