Hello Andrew
Well my guess that the angle of cut was 45 degrees was correct.
You have written proof from the manual that this is the best way to proceed if any one has a frame to " adjust ". I have worked in the heavy truck industry in the UK at dealer level for 40 years [makes me old I guess] and have seen many chassis altered to suit various needs, I have also seen the bad effects of poor modification and unsuitable body mounting ,Twisted frames, Frames cracked at stress points like UN WELDED UN USED bolt/ rivet holes.
I am sure the guys that stretched this original 101 w/ base never thought that it would still be in exsistence 40 plus years on, maybe a compliment to there welding ability.
Regards Stephen
