OK, too much missing info to help You effectively, "a rear suspension kit" tells us nothing that we can use other than the kit apparently has 3" axle tube clamps instead of 2.5".
Putting the power down means having enough traction, IS that the issue? If so SureGrip anything will fix that for most slantys w/decent rubber, and I'm guessing it's an open
diff. at present? Which bolt pattern do you have,4" or 4.5", and do you plan on keeping it that way? Keeping the same rollers & what are they(size/type)? As far as slow out of
the hole goes, a better torque converter is the cure, not gears.....assuming an auto. I tried to make out kickdown rod or dipstick, don't see a clutch rod/bellcrank either, so I'm
going w/auto. Anyways, welcome to FABO! That is a awesome looking greenmachine indeed!