Hi Blaise,
Welcome to the 'Bolt.
I did just that. The first thing I did to mine was do a disk brake conversion and the second was the 3:55 R&P with a complete rebuild while it was down. At that time my truck had an VERY good running 216 and the SM420 like yours.
But there was a little hitch... I had no second gear synchro and I was concerned with the taller R&P I might get into an embarrasing situation if I had to downshift into second. Would she buck or would I have to come to a full stop to downshift.
When the rear end was completed I went for a drive and guess what... yea, nightmare time... middle of town mid-day... traffic is down to a very comfortable 2nd gear speed... and me unable to downshift.
I came to an almost stop and that 216 pulled 3rd gear like a mule... smooth as silk in 3rd gear. Very impressive torque... you're going to love it, especially with a 3-speed full synchro trans.
BTW, Cruising at 50 is a breeze now.
