Originally Posted by Nessism
Get the BB shell faced down to 68 mm. Make sure both sides of the shell are symetric or your chain line will be off.
That can be done but it's a lot of metal to remove and the LBS is going to charge a fair bit to cover the wear and tear on his facing tool. A '70's concord probably isn't worth what it will cost.
The Shimano 68 mm square taper cartridge bottom brackets I recommended will fit with no modifications and a 1 mm "error" in the chain line is a non-issue.