I now have the original 2 speed transmission that works ok but sure leaks a lot. Rather than rebuild I could replace with a better option.
Will a BOP 400 fit? The motor is the original 330. I also see the th350 with long and short shafts. What would be the best option?

 

The short 350 would bolt right in. If you go with a long 1 or a T400 then there are other mods that need to be done at the same time. If your not replacing with another switch pitch trans a new torque converter will be required also.

 

I did the THM 350 swap on my '64 Cutlass years ago. Worked beautifully. IIRC the worst problem was adapting the floor shifter so the quadrant made sense.

 

I'll add 1 more thing for a T350 swap, you'll need to adapt a kickdown cable.

 

Thanks for the advise guys. I have swaped a powerglide for a th350 in a chevelle in the past and added a kickdown cable.
seems easy enough so I'll be shopping for a BOP th350 short shaft.