Right now my '66 coupe has a 289 with a 3 spd that i'm guessing is a c4 with a shift kit. The only problem is that when i'm going around 35-40 it shifts up and down up and down between 2nd and 3rd gear.

I also have another tranny that I'm pretty sure is the original. I don't think there's any problems with it and I'm thinking about swapping them and hoping for better shift points in the original.

My question - will I be missing out on anything great by taking away a tranny with a shift kit?

 

Firm shifts. Check the vacuum modulator on the one in the car now, I bet that's your problem.

 

is it tough to adjust if i can find a tutorial or something on the net?

 

I don't think you adjust them, I think you replace them...

But it could be just the vaccuum hose to that's leaking.