Engine vacuum

[rayfinseats]

Sorry to be such a pain in the ass but I have an 83 4x4 k10 with stock (1968) 350, edelbrock 600 carb and ramhorn exhaust. TH350 trans,
np208 and 2.73 gears. Open 14" air cleaner, short exhaust with duals and glasspacks. Weight 4200 LB.
The vacuum gauge I installed reads about 20-25 at idle. In second gear around town the vacuum is pretty high cruising, 20-30.
On the highway it always seems like it's about 10 and it always seems like I have to keep my foot in it to keep it going. Do you think it's the 2.73 gears are causing the low vacuum? (hence poor mileage)