How are you doing Pete?
My opinion;
You need to get then to count the splines on the drive to see if it is a 19, 3 drive, or a 26 (7?) 3A drive.
They, or a lot of people, don't use 6's because of the cost difference between a 3A and a 6. Double or more for the 6.
3A is a much better drive than a 3. They were built later, the shafts are bigger, they will handle more horsepower. I'm surprised there was an issue with finding props. The large shaft uses the same prop as the 4 and 5, usually a large selection out there. Plus the shorter 4 and 5 leg will bolt to the 3A if you ever wanted to do some really fine tuning with prop height etc.
The price seems high for what it is in this market.
You would need to know the rebuilder 50 hours ago and review what was done with them. Otherwise plan on spending $5 per engine and doing the freshen over. Not worth replacing a $30,000+ engine if the rebuilds can't be documented from a reputable shop.
Chris is right, 850 from 502 cubes is pushing it. May be some Serious boost there which would be tough on the engines without the proper intercoolers.
Check it out. Others on here would have a better idea of its value in this market than I. PM Tank for one, he follows Cigs really close, and happens to own a really sweet one.