1969 B-body here. I didn't cut my steering shaft. I did find a joint with the proper splines for the manual box (adapter) and the correct diameter female-round on the top side. I left the roll pin in the steering column shaft and cut two U-shaped notches in the new u-joint to slide over the factory roll pin. I then welded. I also used a power to manual adapter. This was 15 years ago. I removed it last year and installed a Borgeson. The adapter is still available from several sites.

https://www.ebay.com/usr/rehrenberg?_trksid=p2047675.m3561.l2559