Sill Replacement Chassis Scaffold

[Walshy99]

Hello everyone.

I got myself a 4dr minor 1000 a couple years back and have been very slowly stripping it back to do a full restoration. The floor isn't terrible rust wise but I've noticed a lot of panels riveted in instead of welded but the structure is sound, there's no visible flexing when its shaken as is with all the doors off. However both sills front to back need replacing.

My questions:
How do I do this without the whole thing flexing when I remove one side?

I'm only going to do one sill at a time so how beefy does the internal scaffold need to be to prevent snagging and twisting?

Are there any diagrams available for internal scaffolds or do I just need to guess and hope for the best?

What's the fastest method to remove the horsehair and Tar insulation?

Images are of the offside sill, the one I'll be attempting first.