[Baron Von Zeppelin]

I've never seen anything but #12 heads on 70 WT engines.
Local friend also has a #s matching 70 Formula project car.
WT -12 - 263 carb - open face alternator - yada yada
3spd M13 with AC


As to the gear ratio , your car might provide more insight to Keith_K's study about 3.73 cars.
Yours being a Firebird and showing M21 option is a great reference.
We have been dealing with TAs , that don't show that selection.

On some TA's - 3.73 cars are found with just HD Radiator - No upcharge for Perf Axle.
On other TA's - they show HD Radiator AND Perf Axle on 3.73 cars.
Seems like Keith was pointing towards an Early / Late scenario.

There could also be a factor involving whether it was the M20 or M21 alternate choice factor with 3.73 gears.
But that is getting intricate.
In that case - using your example - it would point to the TAs with just HD Radiator being M21 , and the Perf Axle cars being M20.

But Keith may have already eliminated that parameter.
I don't remember , and I can't access my files/invoices/Bsheets