Slant 6 K-Frame = LOTS of Clearance
I cant say much about fitting the 3G into the Early A bodies, but I had a 340 in my 63 Valiant and I know that it was narrower than the later A bodies. Here is a pic of the engine in my Demon on one of my mock ups. It's tighter than I thought it would be on the passenger side for sure. The drivers side looks great, but the column isn't in there yet either.