The M21 will be a problem as a HT-HG M21 is a Saginaw and the sifter and linkages won't fit in a HZ tunnel without mods. Plus you'll need a HZ-WB TH350 crossmember to mount it.

As Innuendo says the bottom end will be OK but you'd have to run a HX-HZ top end with pollution equipment. You'd need a HQ-WB sump, pickup and dipstick and HQ-WB mount adapters too. There are other things unique to these earlier engines too but the chances of them still being there are slim (like the original water pump).