Looks like the one is a C4(might be a C6) and the other cast iron one is a Cruisamatic. Could be wrong as I can't see the pan shape. As for using another transmission, does your block take 5 or 6 bolts to hold the bellhousing on? Early ones were 5 later were 6, and the 5's are hard to find bellhousings for. If it is a 6, any good later bellhousing will bolt up.

Don