The HD ToTT muncie transmission should also have a drain plug on the lower front left corner.
It might be listed as a Muncie 330 or SM330. I think the 351 is just mr gasket's gasket numbering scheme.
If it makes you feel any better, I too have one of these behind my 70 C10's 350, instead of the saginaw. I discovered this only after I had bought a rebuild kit for the saginaw. D'0h!.
I have not bought anything yet for the muncie, but I have found this site:
https://www.drivetrain.com/parts-cat...d-overhaul-kit
Maybe it will help.
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1970 C10 LWB 350SBC, SM330 Muncie (not saginaw SM326) (on tree), 3.07 open (assumed), PS, PB, Idiot lights