his pipes are 3.5 vband , thats a vband that slides onto 3.5 pipe , a 3.5 vband is about 4.25" measured outside edge of flange . a flange that measured outside aprrox 3.5 (actually 3 5/8) is a 3" vband made to slide onto 3" pipe , ....not sure why he carries it with that 3.5 flange as most 4 bolt turbos up to 76mm are 3" vband except for may be the works turbos
be aware the flange and clamp on the gn1 pipe is different than most 3.5 vband kits you'll find and wont seal properly with other vband kits . i had done one of his 3.5 pipes before and i cut the 3.5 flange off and welded on (in) a 3" vband flange . when i did the 3.5 pipe in my 87ttop i bought a 3.5 vband kit and it didnt work properly with the pipe , so i used the 3.5 gn1 flange i had cut off from the other pipe and had it welded to my 3" exhaust housing , another solution is replace the one on his pipe with one from the 3.5" kit you buy
also be aware that pipe as is fits (the latest one did) but requires a major cutting to the fender liner and most of it ends up in the wheel well rather than the engine bay . if you heat the plastic with a torch it cuts really easy with a razor knife . it did fit with my a/c box but it dont see it working with the drier because the pipeis right there ..so no working AC .
on the pipe i did with the 3" flange it took some major reworking (cut and welding) to make it fit without cutting up the fender liner and i ended up putting a 3.5 vband kit down by the firewall