Rebuild or swap? 1970 Valiant

The slant 6 and small blocks do not share the same transmissions. Their bellhousing bolt patterns are completely different, so that solves that issue. There's nothing "daunting" about building a slant 6, although the manifold assemblies are a little intimidating if you've never removed or installed them, but you're in the right place. Lots of us here can help you along the way if you get hung up.