I've been messing with Tri-Fives, Corvettes, Camaros and other Chevy products for more than 50 years. I've used an awful lot of vendors and suppliers as you can well imagine.
I ordered an M22 for our current project, the '69 Z28, from Riverside Gear in Eaton Rapids, Michigan. Bob Casaday is the owner. The transmission was produced and shipped on the schedule given me. It arrived in its crate and my son and I installed it easily. We wanted to hear its whine and the effect of the new 4.10 rear gear set on this 302's performance. The transmission shifted flawlessly, made the right kind of M22 noises.
But when we parked the car we noticed a significant leak. It was fresh gear lube. The case was leaking at the front.
We pulled the transmission and called Bob at Riverside Gear. He immediately ordered a pickup from my shop by UPS. We re-crated the trans for its shipment. UPS picked it up last Tuesday (3/26).
I received notification today that the repaired (case boring was not to spec at the countershaft) was in the hands of UPS and would be delivered to me on Friday (4/5).
Riverside Gear never hesitated to make good, never queried me about anything I may have done to the transmission. They said: "Box it up, we'll pick it up and fix it so you won't have any additional problems." They did.
I can recommend to the highest degree Riverside Gear if you have any kind of driveline need for which these folks provide parts or service, especially Muncie transmissions.
https://www.riversidegear.com/about-uspaul