What is the difference in a big and small block toploader? I know the input splines are the same so the clutch should not be a problem. Or will a smally bolt to a bb one?

Quig

It's been a while since I've personally converted a big block 4spd toploader to a small block, but I'll try to remember all the details. It's not too difficult at all.

• Input shaft has the same spline but the big block has a shorter shaft. If you attempt to use the big block input shaft on a small block bell housing, you will find that the input shaft's end journal will not engage the pilot bearing in the crankshaft. Therefore, you must exchange the input shaft before it can be used properly.
  
• The output shaft and housings are identical between the two. At least it was on the one I converted. The one thing that needs to be changed is the speedometer drive gear will likely not be correct for the gearing and tire height for your car.

Other than those two items, I found they were pretty much interchangeable. Also, when I've purchased internal components for them, such as blocking rings, gears, bearings and such, the parts listings gave not distinction between big block and small block outside of the input shaft. Thus, the toploader is one strong tranny regardless of what its bolted behind.