Another 273 commando build .. This one will be a "BEAST"

So here is the wild card.
If you are serious about a normally aspirated 273;
IMO, this will require; a solid lifter, high compression, specialty build; and a 5-speed manual trans with overdrive, and geared so you can hit no more than 70mph in THIRD gear, at a fair bit past the power peak, which points to specialty heads.

Here is a trans that could work;the NV 3500 with ratios of
3.49-2.14-1.38-1.00-.73od
With this one you can hit 70@5160, in third with 4.30s, and still cruise 65=2530.

Another is; the AX-15 with ratios of
3.83-2.33-1.44-1.00-.79od
With this you can hit 70@5140, in third gear with 4.10s, and still cruise 65=2620.

With either;
Having passed thru the power peak twice, and ending at 60=4400. You won't need much of a cam to satisfy that.