Slant 6 blocks and tranny matches

I picked up on a slant 6 which had a tranny attached to it out of a 70 Dart. The motor has the numbers 2806830-8 BH. It has 5 freeze plugs and is in pretty good shape. This motor will be going in my 68 Valiant.

The motor that is currently in the 68 valiant has a cracked block with 3 freeze plugs. So I plan on taking what I can off of it for spare parts and dumping the block.
The 68's current cracked block motor has the numbers 2463430-13.

At some point did someone put an older model motor(2463430-13) in this car? If so, what are the chances that i can't use the same tranny?

Would it be best to just match up what I have from the 70 Dart? The tranny and the motor?