60 vs 70 PTO

Jacob, the 60 & 70 are pretty much the same on the PTO clutch parts. There is apparently some small parts that differ according to serial number breaks on 60/70 clutch.

The model 100 came with an 8HP Briggs and the upper part of the clutch is a different part number than the 60/70 upper.
The 8HP JD spec'ed Briggs does has a stepped crankshaft. Hopefully, someone better informed than I can provide the dimensions.

At any rate, the swap is not straight forward as you may already have learned. I once rebuilt an 8HP Briggs block from a Dynamark. It had the same size stepped crank end as the 100 Deere engine. So it worked just fine with the other 100 parts. But most of the 8's are probably straight on the PTO end of the crank.

tommyhawk

Pretty sure the original engine in a 70 has a 7/8" straight shaft. There is a shoulder on it, but its not stepped like the 100 is.

Its common to find the briggs with 1" shaft, could maybe have the clutch bored out and a new keyway broached?

The 70 has a stepped shaft. My 60 has replacement VH70 engine. The short blocks at some point in time were only available with the larger bore.