I have a British 'BPM' Bellows unit with both PENTAX M42 and CANON FD flanges on it -- I use it a lot with my 105mm f4.5 E-Rokkor enlarging lens and it gives good results. When I got my 6x7 outfit in November 1980 I also got teh extension tube set and found it useful, then in about 2003 I got a 6x7 Mk II very cheap with the 135mm f4 SMC Macro-Takumar and found I needed a tube to get really close but the depth of field was very small due to the film size -- here is my Spotmatic with the bellows unit --

PentaxBellows by pentaxpete, on Flickr