I can't speak for Chombi but I would agree with his comment. By not cold setting the frame and forcing the wider axle between the dropouts you are putting a lot of stress on the axle. The dropouts are not going to be parallel. This will cause the bearings to bind up potentially making the rear wheel spin less smoothly. Freewheel hubs will be at risk of bending or even breaking an axle due to the stress. I would suggest you stick with 126mm hubs and 7 speed max. There are many nice, new old stock or used hubsets that you can get in either freewheel or cassette that would work great. Examples include Suntour Superbe, Specialized, Dura Ace 7400, Campagnolo, Suzue, Phil Wood, Mavic, even mountain hubs such as 7 speed Deore XT (where you swap out the axle for the 131mm, to attain the 126mm locknut to locknut dimension). My older 126mm frames are too valuable to try to bend and risk alignment issues so I would never try cold setting on my own, and I am quite mechanically skilled. I say "don't do it".