I stuck an 8 speed cassette hub wheel in my 126 spaced panasonic recently and it slides in with little effort and as long as you get the QR tight, it is solid and the drop outs appear to seat flush. I tested it with feeler guages at the back side of the axle lock nut. I have everything coming for a new wheel set and was originally planning on rebuilding with the original freewheel hub but a very nice Shimano Exage cassette hub (old Trek road bike) came my way for free and I figured why not. I already have a vintage Deore MTB derailleur polished up so this way I can have a 32 cog cassette and I won't have to go with a compact crankset. If I stuck with the 7 speed freewheel I was going to go with a 52/36 front and I would have had a 13/28 rear. Now I can stick with my 52/40 front and get an 11/32 rear.

Before I get flamed, over a year ago I bought a brand new Vassago Fisticuff frame and built a bike from it. Tom spaces the drop outs at around 132 mm so a person has the option to run either road hubs or mtb hubs. I ran Shimano XT hubs with a 135 spacing for a full year and put a couple thousand miles of rough gravel on that bike as well as a week long tour of road and had no issue with slippage at all. All in all I am keeping the rest original such as downtube shifters, bars, etc. If your not concerned about 100% vintage so much try an 8 speed cassette.