Wow: so much fussing and expensive complicated solutions for a non-existent problem. The difference between Shimano 7 and 8-speed cog spacing is 0.2mm, well within tolerances for indexed shifting.

Try it; set up the derailleur in the middle of the 7 cogs on your new wheel. That way, the cumulative error on the ends of the freewheel is less than 1mm.

I once had to customize the spacing on a Suntour Winner Pro 7-speed freewheel to enable a crazy kludge involving Campagnolo shifters and Simplex derailleurs. Possible with custom spacers, but difficult to pull off. Especially with the latest generation of freewheels with hard to remove lockrings.