I tried the 45Cal Huntsman barrel on a modern SB2 frame (which came in 223). It easily fit, a little too easily, what I would term a sloppy fit. There was no vertical play, but it was loose, and wiggled when you tried moving the barrel side to side.
I tried putting the 223 barrel on the 45Cal Huntsman frame, no luck. The modern barrel lug was too wide. I also tried a modern shotgun barrel on the Huntsman frame, and had the same results. I guess if you had the inclination to do so, you could probably "fit" a modern shotgun barrel into the 70's Huntsman frame, by filing the lug down.
I also made a comparison of the lug size, and found the lug length to be the same, but the lug width is I guess about a 1/16" too wide. I misplaced my caliper, so I had to eyeball it.
1st photo is the "Frankengun" assembled.
2nd photo is a comparison of the lug length, bottom view.
3rd photo is a comparison of the lug width, side view. (with help from the wife, holding barrels)