I had the same problem multiple times. This is what i did.
I checked out the tag on the trans to see what it originally came from. IE bigger car usually ment lower rear gears, and earlier shifts for comfort.
Then made sure i had a new red line (adjustable) vacuum modulator. I drove my car and checked the vacuum while the car was at cruising speed. I set my modulator to start pulling in at this measurement. that way when the car is under light throttle it shifts smooth and a lil early but then under heavy throttle it's a lot more firm and a little later.
Normally running anything over 3.42 you have to do governor work. Buy a governor recalibration kit. and start playing with springs and weights. I use a scale and weigh all the weights and compare them to the ones that are original. I also make a graph to record my findings on each combination. also you can remove weight off the outer weight which will help raise your shift points under light loads. inner weight under heavy loads, and the springs separate the shifts.
I went from a 55g outer weight to a 44g. 14g inner weight to a 9g. and 2 step heavier spring. on a 67 firebird 400 th400 3.42 rears. heavy load 1-2 shift 5500, 2-3 shift 5200 rpm.
Also a thing to note is instead of picking 5500 rpm to shift at, you should choose a shift point that correlates with your engine peak torque and hp rpms.