I've got a 1990s vintage Trek 1400 aluminum road bike with 126mm rear dropout spacing. I want to upgrade my wheelset and have a decent set leftover from another project, the problem is that the rear wheel has a 9-speed not-Shimano freehub and I have been unable to locate a 7-speed freehub for it. I know I can put the 7-speed cassette on with a spacer, but that means I can't narrow the wheel beyond 130mm or maybe 129. I've asked some experienced and well respected techs from two LBSs and am getting mixed responses. I've had one say absolutely do not force a 130mm hub into a 126 aluminum frame, an "I wouldn't do it", a couple "should be OKs", and a "no problem".
Just 4mm (1/16") doesn't sound like much to me, but I've always ridden steel, this is my first vintage aluminum frame. If I narrow this wheel as much as possible, is it safe to flex the rear dropouts to run this wheel or am I asking for cracks/frame failure. I'm asking this here instead of the mechanics section because I want suggestions based on a solid understanding of frame materials and geometry. My other option is a frankenwheel born of my 7-speed hub and the better rim, but as the wheelset I have matches and is in good condition, I'd rather not cobble something if I don't have to. Unfortunately, a completely new wheelset is out of the question for now.