Hi Rick:
Be careful now you are crossing into that B body dark side.
If you say there was a letter there was just that it does not seem GM followed it on B body.They increased frame guage in 1966 for sure after having warranty issues in 65 this was enforced by all frame manufacturers.I have 6 frames right now as samples all BB cars, 5 of 6 were 4 piston disc brake cars and all had heavy duty suspension two with F41.
In this mix 4 Budd and 2 AO. I have blasted one of each and and seen no difference in welds nothing.The boys talked about this some also on the impala site. Would be interesting to prove.
http://members5.boardhost.com/impala...175874484.html