2.93/4lb lower here
OP, how are you shifting? I'd guess you weren't banging gears down low with the radials. Power shift 4th? When my bypass was f'ing up, it would usually do it in 3rd or 4th after I would power shift. It would hang open and bleed off a ton of boost, but mine would drop to 4-5lbs. I'd have to see a pic of your vacuum line setup to be sure on that. Belt will also slip the harder you shift. Don't get too frustrated as it's usually and easy fix. What size belt and idler pullies are you running? Have you checked stangshiftergaskets.com(Cobra Bob's site) for the correct belt sizes?