I have the 240mm/5.6 and it's a massive lens (about 900 g weight, 82 mm filters). The 300mm is reported as 1550 g, so it's even more massive!
To be able to focus at all on a 35mm camera, you have to resort to a bellows mount like the PB-4, with extra tubes and a fully extended helicoid, or the PB-6 with its extension PB-6D. Enlargers for those optics must have been huge toys!
Ciao from Massimo