Hi Mark.
this sounds like a tricky situation.
Even if the new crossmember is 4cm further back - I believe it might limit your possibilities to move your seats back. Either way, I would recommend putting in new seat floorboards and test seat location. I decided to replace the 65mm high wooden seat supports with 50x50mm metal square-tubes - something I can really recommend as the seats will be slightly lower and 70mm further to the rear. (See below)
Additionally, the original cover for the leaf springs and the Armstrongs will not fit anymore.
Thinking about it - personally, I would tend to modify the new chassis and make a new crossmember.
Regards
Tim
![[Linked Image]](https://fixingc6622.files.wordpress.com/2021/02/steelsupport-testfit.jpg)