2nd-3rd went south

If reverse works (I say this because 3rd and reverse are tied directly via the front clutch assy) and your only problem is a sloppy 2nd to 3rd shift it could be something as simple as a front band adjustment or throttle pressure (kickdown) linkage adjustment. If it's not adjusted properly the shifts will be mushy. If the trans. is still in the car first verify that the throttle pressure linkage is adjusted so when the throttle is pressed all the way the linkage is pushed back all the way. If that's OK check and adjust the front band. This adjustment is located on the drivers side right above the shift linkage. You need a small inch lb. torque wrench to tighten it to 72 inch lbs. then back it off 2-1/2 turns then tighten the locknut. If the trans is out of the car you can still check the front band adjustment by doing the reverse order of the adjustment I mentioned above and seeing how far out of adjustment it is. If the adjustment seems close next step is to remove the pan and see how much friction material or other debris is in the bottom. If there's alot it's probably time to rebuild it. Also if it smells burned that's not good either. If not much trash is in there install a new filter and adjust the front and rear band (rear band adjusts same way as front, tighten to 72 inch lbs. but only back it off 2 turns then tighten the locknut). Try this and let me know how it goes. Feel free to PM me.

Sorry I'm not a manual trans. guy. Can't help with conversion tips on that other than I know A-bodies have a trans. that is specific to them.