Rarity of the Flash-matic transmissions

They aren't particularly rare. It is, however, a 50+ year old transmission. One in good working condition is usually in a car that has been taken care of. Once out or parked they deteriorate fast -- any auto trans will. They need warm circulating oil to keep the seals from drying out. It's hard to get someone to work on them any more, but not impossible. 

Not really. I've seen them stored in HD contractor clean-up trash bags -- the thick ones. As long as they are in a dry area you should be fine. I'd consider any trans that has been out of a car for a long time a rebuilder core anyway. It would at least need a seal kit installed and the valve body cleaned. 

thanks for the information on the converter, did not know that.

While technically it should work, bolting up may be a problem. You'd have to compare the bolt pattern where the trans bolts to the bell first. If those are the same you can bolt the six trans to a V-8 bell (pre 72 six trannys use a different bell than the V-8, 1972 and later six and V-8 bell bolt patterns are the same, but different depths). Then you'd need to compare the depth of the bells. If the depth is the same check the torque converters.  

After all that is done, you need to drive rather sedately. The six trans isn't as strong as the V-8 trans. If you just cruise around it will work fine and last. Drive a bit aggressively and it will work for a while, how long depends on how hard you drive. Expect a life reduction of about 15-20% even if you rarely "get on it" though. If you drive it a bit hard life could be reduced by as much as 50%. "Get on it" a  lot and it may only last around the block.

Parts are not easy to find for them. (Hard parts) so the cores can be valuable for that.    I have a 1969 Rambler, with the M36.