Tim,
Did a little checking and found out an O engine will work on a 70. The only major difference is the O oil drain is on the side, and the 70 oil drain is on the bottom.
You would have to drill a hole in the side of the 70 frame and run a nipple out to the side.
Both dipsticks are the same height.
With all pulleys, baskets, pto, and the carbs off, both are identical motors except for the oil drain.
Here's some pics of both motors. It's hard to tell from all the grime, but they are the same.
As for the pulley on the S/G, they are just a little different.
As Lew mentioned, the S/G pulley
on the O engine does not have a basket for the pto. You could use the O pulley, but you could not have a pto on the front.
In short, if you have the pulleys, basket, and pto off of a 70, they will fit on the O engine.
Original S/G pulley:
70 S/G pulley along with some dust: