You won't find an aussie 4spd M21 behind a 6cyl except in very rare cases (like VB rally cars), M21 Saginaw was fitted to a rare few HK-T 6cyl cars. Almost always the 6cyl cars got an M20 Opel or Aussie or some commercials got M22 Saginaw or aussie.
You are most likely talking about Aussie 4spds in HQ to WB though. In which case 6cyl and V8 boxes are different by the length of the input shaft and the cover over it (that the clutch thrust bearing carrier sits on). The shifter, crossmember and speedo cable are the same and the box sits in the same location, but everything else is different: bellhousing, clutch, flywheel, clutch mech, tailshaft and diff. How much differs in the clutch mech (and pedal in some cases) depends upon which Series you are talking about.
What exactly changes between auto (assuming trimatic) and manual (assume aussie 4spd) also depends upon the Series, but in all cases and for the same engine it means flywheel, flywheel bolts, spigot, bellhousing, dust cover and bolts, gearbox and bolts, shifter and linkages (correct one for bench or buckets or console), shifter boot and retainer, correct hole cut in floor if not there, manual pedals (correct for body), clutch mech (correct for series), Xmas tree for manifold (auto has vac source), and reverse lights harness and detent bridge. The rest that is Series dependant is stuff like: carby, exhaust, pollution equipment, even the engine itself in some cases.
In the end it is often cheaper to buy a whole donor car of the same Series as yours with the box you want and sell off the remainder.
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