Hi
I don't think you will be able to bridge 5mm with weld, if you had say five joints you could maybe bridge 1mm on each, better to make the frame fit what you have now, or maybe weld in a 5mm spacer plate somewhere. Done properly this could work.

Yes machined aluminium tooling plate is very good and ideal to use as-is.
Yes, fit heavy adjustable feet, always useful, but not the weedy ones with M8 threads ;)