5.9, max safe compression?

Or just put in a different cam with a later-closing intake. Unless that too has been purchased. Or unless the bigger cam puts the power somewhere that you don't need it to be.
Or get fresh cold ait into the carb,or a smaller carb, or a spreadbore carb, or run different fuels between the primaries and secondaries,or stage the secondaries later. Or install a higher TC, or bigger rear gears, or a lower first gear in the tranny. Or there's water injection at large throttle openings, or more ethanol in the gas, or even ethanol injection. Or just don't floor it,lol. Or if 32* gets you out of detonation, run 32, or just bring full timing in a little later. My engine does not get full timing until 3400, so I can run 87E10 @10.9 Scr. Do you think I have ever noticed a power loss?NO! At 2800 the timing is only 28*. I have never noticed a power loss.
While I still believe 10.9 is too much for iron, go back to post #28; 65 Dart 904/3.55s/fun at car shows.
Nowhere does it say heavily loaded or racing. I'm betting you will be running skinny tires too;probably 245s.
I was unable to find where you listed the stall speed so I assume she's stock; 1750 to 2200. And you are at 1000 ft.
In your tune, you have the ability to generate any timing curve you like, and at PT it can be massively below the detonation threshhold. So the only worry will be at WOT too-heavily loaded or at too low an rpm. So don't floor it in 4th at 2000 rpm!, for goodness sake' Who would do that?
It's a 65 lightweight-A; you are already ahead of the game some 400 pounds. You are 1000 ft elevation in your favor, and it's not for racing.
I say go for it, I would try it, and tune the heck out of it.
With a 64* ICA, and 1000 ft,I get

Static compression ratio of 10.9:1.
Effective stroke is 2.79 inches.
Your dynamic compression ratio is 8.52:1 .
Your dynamic cranking pressure is 173.47 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 8.52 :1.
V/P (Volume to Pressure Index) is 148

This appears to be a half a point too high in Dcr, It might cost you 2* of power-timing at 3200rpm, maybe 4* at 3600, who really knows and will you ever feel it? I doubt it with 3.55s and 300 to 400 less pounds,and skinny tires.And don't forget, you can stage the secondaries to come in a little later. The primaries will make plenty of tire frying torque all by themselves, and if they don't, a sure cure is a 2800TC. Ima telling you; that cam will shred both 245s with a two barrel; A really little 2bbl; A 600 Holley vac secondary with the back barrels wired shut,even!
I ran a 270-280-110 with 223@050, And it could do that with ZERO help from the TC. Just flat-footing it. And that @ 3650 pounds;me in it.
So, as a streeter, you have plenty of after-the-fact options.
I say give 'er.
What's the worst that could happen?
Let's see;
it's not the 2800, that would be a great thing.
it's not the 3.91s or the 2.74 low gear tranny. No, those would be great too.
it's not the TQ, no sir, those are awesome.
it's not the 180 stat in place of the defective 195 that cranks it up to 205.
It's not the full-timing delayed to 3400, nope, the tires are still spinning.
So what is the very worst thing.
Oh I know, the worst thing would be if you go deaf before the tune is in,and you cannot hear the marbles. But I'll tell you a secret; KB107s will survive quite a bit of that, a little atta time. They may not be diesel pistons, but they are not ticking time-bombs either.