"Aussie 4speed M21 which are the original 'boxes for our C3s"
If your C3 came with a Aussie 4 spd you got ripped off.
"M20= 6cyl Aussie 4sp, M21= V8 Aussie 4sp,
and M22= Muncie."
GM USA use the same no to identify Muncies ;
The M-20 wide ratio,
the M-21 close ratio,
the M-22, Extra heavy duty close ratio"
The Australian GM manuals were never used in US cars, Muncies , Saginaw and B/W T10 and Super T10 were OEM GM trans.
Given the cost /availability of OE type 4 speeds in Oz (Muncies were last used in '74 so old technology), the above suggestion to use a V8 5 spd would be the better option.
Very little difference in size/ installation , readily available/ cheaper and far superior shift mechanism as all internal , not external linkage.
If you are prepared to risk explusion from the club ,Dellow and others makes several kits to put "Japanese" 5 spds behind Chevs .The Supra box is very popular as a V8 conversion and well up to handling the Hp of a street smallblock.
FWIW the Super T10 used after '75 (the 4 spd in the C4
4 +3 box ) is generally considered to be stronger than a M22 without the noise.
Watch "Two Lane Blacktop " and listen to the noise the gearbox in the '55 Chev makes , not a comfortable street trans