I busted the bolt off on my NP205 shifter one snowy evening ,that I put in a 77 GMC ,originally it was from a 72 vintage truck..the bolt was a special shouldered type,similar to one on a lawn mower wheel...if I remember correctly it was a 7/16" coarse thread,and had a 5/8" or so shank where the shifter pivoted on it..
I ended up drilling the broken bolt out ,the drill drifted off center though,and I ended up tapping the adapter housing for 1/2" x 13 thread and used a 1/2" boltlong enough to bottom out on the adapter yet still have enough length for the shifter to rotate freely-- with a needle bearing I happened to find that was 5/8" outside and 1/2" inside diameter to take up the slop between the shifter and the bolt..
I had to add some washers to take up the excess end play ,had to use a bolt slightly too long,so the shifter wouldn't just tighten up against the adapter housing when the bolt was tightened...I could have used threaded rod I suppose,but felt it was too soft and might bend or break too easily..bad enough the stock bolt did,it should have been designed with a brace on the head end to prevent it from being bent in the first place in my opinion..
If you look hard at places like McMaster-Carr you might find a suitable shouldered bolt...I doubt chevy dealers would still list one..