Which engine would be easiest for my swap?

I don’t know what transmission yet. The dart has a slant six and the transmission is great it in, but I’m assuming it won’t work with the magnum. I’d like to get my hands on an overdrive transmission if I could find one at a reasonable price. I’m planning on putting a keep Cherokee 8.25 in it with 3.55 rearend. Open to transmission suggestions.