2.76 vs 3.23 vs 3.55

If yur still running the stock convertor; then just get a higher stall; that will make way more difference to your take off.
But honestly, 23.5 x15s in wheelhouses that should accept 28s? And your 67
(Modifications: RV cam , zero decked, 4 bbl, headers, true dual exhaust (no cross pipe), and an open diff), can't even squawk one of them?
I'd guess you don't have much cylinder pressure so that's where I'd start.
What you want is; 185psi, lol, that will wake that 318 right the heck up.
Second best with closed chamber 67 heads would be ~160/165psi.
If it can't make even 135psi, well, put a 2800TC in it and, just drive it.
If you're at over 1000ft elevation, stuck with low pressure, yur screwed anyway you look at it.

BTW your 23.5s are already ~17% shorter than 27s so that makes your cruising rpm similarly 17% higher. 3.23s would put it 17% up so, you are already running nearly the equivalent of 3.23s and 27s, and you still can't squawk even one tire? that is bizarre, something is wrong with your combo.

your Cruise rpm@65mph should be;
with 2.76s and 23.5s; about 2570@zero-slip, perhaps 2650 on the tach.
with 3.23s and 23.5s; about 3000@zero-slip, perhaps 3100 on the tach
with 3.55s and 23.5s; about 3300@zero-slip, perhaps 3400 on the tach.

From all I know, every 2% increase in cruise rpm, will lose you 1% in fuel economy with no other changes. So then, a 12% increase in cruise rpm might be a 6% loss in economy , If your Sport is making 18mpgs with the 2.76s, then expect 16.9 with 3.23s, and 15.7 with the 3.55s . This is in steady-state hiway usage. Around town, it will be worse, because of stop and go, plus accelerations up to speed. a lot worse.

I would stuff a 2800 convertor in there in a heartbeat.
Well, I'd also get rid of those 23.5s lol.
225/70-15s are about 27.4, and so, will reduce your current cruise rpm to
23.5/27.4= 85.77% or 2370rpm. To get the performance back to where you now are, would require a gear of 2.76/85.77%= ~3.23s to where you are now.

Here's the trick.
If a 2800 TC gets your engine up the torque curve say 50 ftlbs, then at 2800rpm that is 27horsepower. That is the cheapest, easiest, fastest way to get 27 stinking normally aspirated horsepower, down low, bar none.
But it doesn't solve your dead duck 318 that can't even squawk one tire, at the current stall.
Again
I would start with a compression test, and a timing system evaluation.
Maybe your cam has gone flat.
maybe the rings are bad.
maybe the distributor is seized.
But for sure something is wrong.