I just finished a 4 cyl to v8 swap on an 1988 automatic mustang and I can't get th car to idle at less then 1000 rpm and I only have about 14" vacuum at that speed. If I step up engine speed to 1500-2000 rpm vacuum will go to 18" and stabilizes there. The donor car was a 89 5 speed car. Does anyone have any ideas on what could be going on. Any help would be very helpful. Thank you.

Specs-
Stock 302 block
Flat top pistons
E-303 cam
Stock upper & lower intake
70mm throttle body and egr spacer
Fender mount cai
Oversize maf housing
A9l computer
Aod transmission
BBK shorty headers
Under drive pulleys