Don't think the old flywheel will fit, and you probably wouldn't want it if it did. Why? Because six cylinders have smaller diameter clutches that a small block will burn up in short order. When you get another flywheel, have it resurfaced and then get the biggest diameter clutch you can to go with it. When you go to the parts store, as for a clutch "kit" that will include a new clutch, pressure plate and throwout bearing. Typically you could ask for a kit from 60-72 chevy truck with a small block and the same transmission you have.
Rather than fighting with the front mount for a small block, you could get the mounts from a later small block truck 62-66 - they should bolt to your crossmember. There might be a few problems with this, because I'm not sure how things were on those torsion bar trucks. Check it out for yourself.
If you have a floor shift transmission (SM420), it should have an 18 spline input shaft, which is very common. You won't have any trouble matching it up to a clutch.
I beleive the column shift have the same spline count, but I'm not sure.
