See if you can send it back, it's not worth trying to mess it up to plug those holes. If you don't plug them, lots of exhaust will be coming out of those holes.
1986 Ford F-250 XL 4X2 351W H.O. Weiand Stealth Intake, Comp XE250H Cam, Comp Magnum Rockers. Summit/Holley 600 CFM 4010 4V Carburetor. 130A 3G Alternator. PMGR Starter. 10.25" Rear With Open 3.55:1. 237,000 miles.