Dads53,
Yes, the passenger-side mount is at least a 1/2" further aft. I recently put a 1954 261 in a street roadster and discovered this difference. Should not be a problem as the frame-to-engine mount brackets needs to fabricated anyway.
I don't know about a longer yoke for the 420, but it seems that making a longer driveshaft would be easier than cutting into the firewall.
Hoyt
