I love shooting 6x6 medium format, and prefer square format images. I thought it might be fun to attempt to modify a 35mm SLR to take square images on 35mm film. I assume the masking of the viewfinder and film plane will be quite straightforward, but I'm unsure as to how I might be able to go about adjusting the frame spacing. Can anyone recommend a particular 35mm SLR I should seek out which has easily adjustable frame spacing? I have a Pentax ME Super and a Canon Rebel X which I'm willing to modify, although I imagine I should get a simpler, fully mechanical camera. Any tips on the general workings on film advance mechanisms or how to adjust them?