Don't block the heat cross over in the intake. It helps vaporize the fuel for better milage. If you had a wideband O2 you could get the cruise as lean as about 16:1 or more. My 383 in my Luv gets 18mpg and runs 10.80's. I drilled and tapped all my bleeds and restrictions in my Holley 750dp and recalibrated it. I cruise 60mph @ 3000 rpm.

Combo:
383cid 11.2:1 CR 3.750 stroke X 4.030 bore
Dart Iron Eagle 200cc/64cc home ported heads
Victor Jr. Intake 1" super sucker home ported
Holley 750dp recalibrated 13.5 idle AFR, 16:1 cruise AFR, 12.5 WOT AFR
Comp solid roller 288/296 252/258 .630/.630 106/104 1.6 rockers
MSD billet distributor 6AL box 18* initial 34* total @3200 rpm No vac.
Th-350 4500 stall 4.11 gear 30X13.5-15 tires