Better acceleration?

First off; 75mph at 3000 rpm points to 3.23gears with about 24" tires.
Your problem is not gears, rather it is the 4000 ft; and you can't do a darn thing about that. If you want to keep that engine, you will need to super/turbo charge it, as per posters 4 and 7 who are are right on.
Else get a bigger engine ......

The optimum pressure for best gas is about 165psi, which in your 225, with a stock cam or any cam with an Ica of 48*, happens at 9.0 Scr at sealevel.
By 4000 ft, the pressure is down to 143psi, according to the Wallace Calculator. That is a loss of 22psi and is the equivalent of, running;
at 8.0 Scr at sealevel, or
at 8.3 Scr at 1000 ft.
Only a turbo/super charger can pump that pressure back up; or a purpose built 9.9 Scr 225engine, that you better not try to drive at any lower elevation.