So you want to buy a 4.8 retainer, bore the bellhousing to match the 4.8 retainer, and hope that all the machining is spot on? No thanks. I've done this too many times to worry about the 307 bearings. By the time I tighten everything up and have the stronger cast iron case with 90 wt gear oil, I'm good. I haven't worn one out yet, and I've been running one since the late 70's. I can simply slip my O/D in place of the 4 speed, hook up the shifter, and am good to go with easy stock parts that I can put back to stock whenever I want.
