old school 340 cam help

If you go back to that big cam deal, with iron heads, consider the 284/292/108 and bring the Scr down achieve 160/165psi so you can run pump gas. This will need about 10.4/10.5 Scr. You will lose a fair bit of power on top, but the bottom should stay hot enough, and you can use pump gas, and she will drive quite a bit further on a given quantity of gas. For a streeter, loosing power at 5700 is not a big deal, cuz you usually only go there once on the way to 65 mph, and the tires are spinning anyway, even with a smaller cam.
In your case, you will get there twice with 3.91s, but the second time you will be at about 6100 in second gear, and of course still spinning. This means that you could probably drop down at least one rear gear to 3.73s, and hit 65 at 5800, a pretty good match for that 284 cam. This will add yet more mpds (miles per dollar) .

If you wanna keep the 12.5Scr, consider aluminum heads. I have run my Eddies at near 200psi on pump-gas. Your 12.5 will run that at least, with the 292/108. The aluminum heads might bump your 12.5 up a tad; so perhaps a combo of Dome removal and al heads may be needed. I ran my 367 at 11.3Scr with that 292/108. Did I mention on 87E10? lol. I think she was making 185psi.
So, for you,11.5Scr and an ICA of 58* on that 284/292/108..... will get you 200psi on the nose..... With a killer VP of 167. That is more low-rpm performance than a 68 440 Magnum. You can run 3.55s with that, maybe even 3.23s
I can't say for sure if that will run on 91 gas, but it will be close.
I have more power than I can use ,now at 180psi/VP of 153 (276/286/110* cam), and I highly recommend that 185psi...... cuz it runs like a scalded cat on 87E10..... 93mph in the 1/8th at 3467 raceweight, spinning most of the way, it was told to me.