Rear upright tube - tolerance

[Bonzo]

I would imagine that the wall thickness would be the most important thing

You'll almost certainly be able to make allowances for the extra 3mm OD when you fabricate the jig

Just measured a piece of standard UK spec 5" pipe

The wall thickness is 4.9 mm & the oD is 127.5 .... As steel sizes are nominal, I would not be surprised to find a piece of 5" pipe with an OD of 130mm nor as little as 125mm

If the wall thickness is there, I would say, go for it.