I just went through this exact issue to get my Hampton Blower Intake Manifold to work with the AFR 225 heads (Fel-pro 1262R gasket) on my 408W.
I had a local custom welder add about 5/16" of weld (TIG) in 4 spots on each side of the manifold flanges over the intake runner ports. He said it was a very simple job and only charged me 1 hour shop labour charge.
Then I took it to a machine shop and they planed it smooth (another 1 hour shop charge), turned out really nice in the end.
FFR#7314 - MK4
8-71 Blown - 410W, TKO600, 3-Link, Konis, 3:27 rear, Stainless Sidepipes, Speedhut Gauges, Hoops & Overriders, 295's on 15's, PS, Hydroboost