‘66 Barracuda

Just remember, on the street, horsepower ain't everything; don't get caught up in the numbers.
The biggest problem with a small-engine streeter, believe it or not, is a lack of gears.
A 5-speed would solve most of our problems, or a 4+1overdrive.

Take the Ax-15 manual tran, for example;
The Novak Guide to the Aisin-Warner AX15 Transmission

The AX-15 ratios are 3.83-2.33-1.44-1.00-.79
These are almost ideal for a small engine with a wide powerband...... like a 273/318.
You just put second gear where you want it, cuz most of a streeter's life is gonna be in second gear. and watch the rest unfold.
Say your 273 is stock. it will power-peak at about 4000, and you can probably shift it at 4500. The torque peak will be at about 2500..
It will pull well from 3000 to 4000, so that is where you are going to want to be. In 2.33 second gear 32 mph will need 2.94s, for 32=3006rpm by the math, and 40 will be 3760 and 4500 will top out at 48 mph.you can't hardly get a better second gear than that. The starter gear will be
3.83 x 2.94=11.29/ funtastic. and in .79overdrive, she will cruise at
65=2070 really nice.
Third gear will cruise at 35=2030/ super nice

I'm not suggesting that you drop everything and install an AX-15! Not at all, I just used this as an example of the gearing problem.

The AX-15 ratios are 3.83-2.33-1.44-1.00-.79, and with 2.94s, the road-gears are
11.26-6.85-4.23-2.94-2.32 ;the important ones to see are the 6.85 and the 2.32, as shown above.
My question is; can we duplicate those with an A904; 2.45-1.45-1.00?
Well,l I think you can get the 6.85 in second gear with 3.91s at ~20% TC slip; I get 6.80; so that is doable. First gear will then be 9.58 times the TC ratio , which could be 1.8 at zero mph diminishing to about 1.4 at shift rpm, so 17.25 to 13.41; so that will be a blast.
Third will be 4.30@10% slip
and that's all. lets line them up
17.25-13.41-6.80-4.30
If you had a lock up you can add 3.91, and lets average first gear; So
15.33-6.80-4.30-3.91 compared to the AX-15 below it
11.26-6.85-4.23-2.94-2.32
That's looking pretty good, except, without an overdrive, 65=3674

But here's the deal I chose the 3.91s to make the numbers, so the 273 could be the best that it could be.
If you installed a 318 which is ~16% bigger, then you can easily take out 16% rear gear; so now that would be 3.23s; and the new numbers are;
14.24 to 11.08-5.62-3.55-3.23 in loc-up; well now, that's a whole bunch better even 65=2880. Even tho the numbers are all smaller, by the time you multiply them by the engine torque, this 318 will be every bit as peppy as the 273 was.
But It gets better. You now have a 318 with waaaaay more torque that the 273 could only dream of. What if we installed an A999 with the wider ratios and matched up the second gears. The A999 ratios are
2.74-1.54-1.00, and the roadgears with 2.94s are
14.51 to 11.28-5.43-3.23-2.94 and 65=2620;compare to the A904
14.24 to 11.08-5.62-3.55-3.23 and 65=2880
You can see how I did that by swapping out rear gears to match up second gears.
Is that the best option?
Maybe, but I got another one up my sleeve. All these combos are designed to maximize your second gear WOT power.
We could do a similar thing by putting your 273 on or after the torque peak with whatever rear gear you have and again at 32 mph, by adjusting the stall-speed.
Say you currently have 2.73s, and a 77" tire roll-out just like in all the other examples. Your rpm at 32 mph then, in first gear, will be 2930 at zero slip, say 4100 at WOT So you don't need any TC help at 32 mph. But on the start line with a starter gear of just 6.69 your 273 is gonna need a bit of help to squirt away from the light. So, swapping out the 2000 TC for a 3000 will put your engine on/or close to on, the torque peak, and that 273 will really jump off the line.... because;
first off, the Tc is up on the TM peak of say 1.8, so that lowly 6.69 might be acting like 12.04 at zero mph. and
secondly you 273 might be putting down 20 ftlbs more at 3000 versus 2000!
So here we have same 273 engine, and same A904 trans, and same stinking rear gears, acting with a whole bunncha more attitude, and all it took was a higher stall TC. Will it compete with a 318?
Well, no .
But you said;
Not planning on any racing, and
just a nice cruiser..... with a lil extra power.
IMO
A "hi-stall" TC, is the cheapest, easiest, and fastest way to achieving a performance goal
A 4-bbl/ free-flowing exhaust, would be next, and
then Gears ..
But if it's tire-frying power you're after......... that's a different combo