Room for a Clutch-Fan in a '65?

Is that a factory 273?
rated 9/1?
but now with .039 headgaskets? which would then make it an 8.4 Scr engine. So now,
with the factory cam, at 8.4Scr, and say 400ft elevation, that should make about 147psi with the factory 273 cam, that had an Ica of around 48*, which you-know, was pretty modest.
So Now;
with the unknow-spec Commando cam, and the 8.4 Scr, and say an Ica of just 60*, The Wallace predicts a pressure of just 134psi .................. but you measured ~150 .............
So then, I would do a new compression test with a different tester, with the valves correctly lashed; and if it comes out the same 150psi, then I would check the cam-timing............. cuz at 150 psi, it shouldn't be that lazy in a Lightweight-A, unless the ignition timing curve is real lazy, or, unless, are you perhaps expecting too much?.

IDK anything about a RacerBrown Commando cam, but if the ICA is anywhere near 60*, then your pressure is more like 135psi, which is way less than any solid-lifter 273-2bbl ever was, and couple that with a 2.94 rear gear, and that means that on the 1-2 shift, the Rs are gonna fall into the basement (62%), and then the poor 273 might have a heckuva time trynta climb up outta that hole.
The thing about this pressure is, that if it less than the 2bbl stock-cammed engine was, then it is gonna feel like a smaller engine until the cam starts to wake up ..........exactly as you say, taking time to come to it's happy place.
If this is right, then, in order of cost, there are only four things that you can do, to alleviate this time-lag, on this particular engine;

1) Check the timing curve, looking for headroom to increase it.
2) shed a few hundred pounds off the chassis.
3) install more rear gear; I mean 2.94s are fine with a 3.09 starter, but as soon as it hits 2nd gear, it's practically over.
4) increase the cylinder pressure, looking for about 160/165 with tight Q, closed-chamber, iron heads.

BTW
The Wallace predicts a V/P index of 88 with the Ica of 60 degrees as noted above at 134psi. This is about the same V/P number that a stock 225 slanty makes (~87). So then, that's what such a 273 will feel like.
Increasing the pressure to 150psi, the V/P climbs to 99, which is plus 12.5%. By 165psi, the V/P has risen to 109, another plus10%, but nearly plus24% over the starting 88V/P
For comparison, the stock 7.8Scr 318 makes a V/P of 111@133 psi, thus being just ~2% stronger out of the gate.
Read about V/P here
V/P Index Calculation