cab rebuild

can anyone tell me were i can get ,ablueprint for a 6718 1977 vintage. as need to replace most of the lower half of the cab
due to tin worm. the steel is 35 by 50mm, but i am told the nearest you can get vnow is 30 by 50mm? any info welcome ....

I am replacing parts of my cab frame also. Water leaking between the guards and frame seems to lie around under the cab soundproofing and rust results. The plastic piping does not seal the joint between guard and frame so replacement of that with a better sealing system is intended.
The frame on my 7045 is 50mm x 50mm and patching of parts looks straight forward. Guards also need some rebuilding particularly around the tail light area with some double skin water traps.

The rubber seals for the glass deteriorate particularly at the joints letting water in which seems to corrode the small upstand carrying the rubber glazing moulding.

My cab is a BK6911 - UBK type with the wide top section on the doors.

Making up odd tube sizes is possible but distortion with welding has to be managed. Light gauge angle iron can be cut to size and two pieces welded to form a rectangular section. Not a good approach for longer pieces.
Similarly 50mm x 50mm tube can be sliced with a thin cutting wheel and welded to for to form 50mm x 35mm but a bit of a desperate measure.

If your 50mm x 30mm is a heavier wall thickness than the original then it may be able to do the job.

Good luck