1966 dart eng/trans plus misc. parts

I found 2 possible candidates to replace my slant 6 that has the cracked block. The first one is a free \6 from a 66 Valiant. Will this one mount on my 68 Valiant with no issues? I heard some where that the earlier model \6 had a different arrangement or something with the tranny.
Or I found a newer 5 freeze plug engine that came out of a vehicle that was running propane before. The numbers on this motor are 4343 730. The numbers on my block are 2463430-13. Can I use this one? I heard the 5 freeze plug motors are newer and better built.