Low RPM jerking

Hey everybody, I recently purchased a 1990 Mustang 5.0 LX coupe. It has been Board over headers straight piped ex BBK 70 mm throttlebody Plenum spacer cobra intake . I have redone the vacuum lines. The car has been sitting for a while at wide open throttle. It seems to be OK but you can still feel like it’s holding back a tad. But higher gears trying to cruise jumps back-and-forth like it’s missing spark I got my timing set up about 14 cannot figure this out also has a E303 Cam someone please help!!!!
 
E303 bucks at 1,400 and below. This may be somewhat tuneable, as I've had it where I could lope along lower when I played around with things, but after more adjustments, it's bucking there and lower again. If you're at 1,500 RPM and above, then I'd say you're probably right about your concerns.