Ever think about going to a shaker setup? With the hood swap out and painting it, you are looking at around $5K. The fresh air setup was worth about .1 in the quarter mile times. Gives you a nice free breathing air cleaner, and whats more of an impact is it draws in fresh air. Thats the big difference, not the air cleaner itself. The air cleaner swap by itself is not that much improvement, no matter how restricting those stock air cleaners look. Look at our SRT-10's. Changing from the stock air box to a K&N or some other aftermarket setup picks up like 10-15HP. Those stock SRT-10 air boxes cant be more restrictive than the stock 440 air cleaner, so thats what you'll pick up with that swap. Getting the motor fresh air is a big help, and it would remain stock looking. You could also change over to a T/A style setup. They really work well, and in 1970 if you ordered a shaker hood you may have gotten a T/A hood setup due to production problems with the shaker setups on the challengers. The T/A setup is cheaper than the shaker, i bet you could do the T/A setup for around $2K.
After all that though, i'll say i dont like the T/A setup on non trans am cars...just does not look right IMO. NOTHING looks better on an e-body than a shaker hood. But again, it's your car, do what you want.
The intake /carb swap can be done and the car will still look stock if you paint the intake to match the engine...I'd go that route.
As far as gear changes, if you have the OD setup, I'd go 4.30 in the rear. Changing to 4.30's from 3.91's is a nice stepup, and with the OD you can still stay driveable on the highway.
Just my input here, again, it's your car and it looks great no matter what you'll decide.