340 street build help from experienced input

At 10.5Scr, and with an Ica of 63*, your pressure is predicted to be about 145psi and the bottom end is weaker than a 318LA. For me, this will suck with that 9.44 starter gear, all the way to at least 3000 rpm/25 mph. In Second it will suck below 35, and in third below 48, and forget passing in Fourth.
So the cures for this are More compression, A shorter Ica, or bigger rear gears.
The next smaller cam will get you about 5 psi (now 150psi)and a bottom end that feels more like a 5.2M.
2.5cc less gasket will get you 10.8 and another 5psi, now 155psi.
switching to a 108LSA cam will get you an Ica of 58*, and 158psi and a much stouter bottom end.
So if you wanna keep this cam, I highly recommend a 3.09 low gear, and probably yur gonna need 3.73s or more. But you can at least try the 3.55s (with the 3.09low), cuz I mean, the 3.09 offers 16% more torque than the 2.66.. But you will have to rev it a lil higher before shifting into second, cuz what it gains in first, it loses on the 1-2 shift. 3.73s will get you 5% back, and 3.91s would get you 10%. Do not even consider the overdrive box.

But at your altitude I would highly recommend a solid lifter tight LSA cam, to get the pressure up to 160 at least; Then you can run the 2.66x3.55.
A solid lifter cam might have longer acceleration ramps, but from when the lash is taken up to .050, tappet rise, the solid will have fewer degrees; soooo at low-rpm, she will act like a smaller cam, than the same .050 hydraulic cam. The difference could be more than six degrees in total, so on the lifting side, the Ica would be about half of that, smaller; which is good for ~5psi pressure. Which is a lot! with a minor penalty of annual lash adjustments. But the added expense of adjusters.
If you did go to solids, I suppose you could go one size bigger at .050, but at your altitude, I wouldn't. 340s were never really known for torque with the 268/276/114 factory cam, and at 6000 they would be miserable, with a predicted pressure of just 130, and take-off power like a 273. So losing pressure would be a bad thing.