New guy needs help MC1 Transmission seals

[HuBarlow]

Hello! New to the forum, but not to 67-72 Chevy trucks. I could use some help, please. I am searching for transmission gaskets for the Muncie MC1 (Chevrolet build code) or the M13 (Pontiac build code) Heavy Duty 3 speed transmisson. I keep looking online and all I find are parts for Muncie 4 speeds or the 318/319 Muncie 3 speeds. Not even sure what the codes are for Muncie itself, and that might be the problem. The transmission is from a '72 Chevy half ton truck with a 350 / 3 speed. It seems that GM used the Muncie HD 3 speed in the performance cars with big blocks and the trucks with v8 engines. Everything else got the light duty transmission unless specifically ordered that way by the customer. Any help, information, and advice welcome. Thanks!