Just about any GM 4 speed granny box will bolt to your bell housing. The SM420 (reverse left and down) was built until 67, after that the SM465 (reverse right and down) was used. Best thing to do is find a junkyard and measure the length to compare. Also, you can unhook your driveshaft and see how much fore and aft movement there is in the slip joint. Just don't run it clear to one end.
On the antifreeze, I would flush all of the coolant out and run only water until either the problem is found or it gets too cold. I don't know which end of Maine you're in, but you should have a few days left before the snow flies!
