I think your math is a little off. I come up with .196 sq in for the cross sectional area of a 1/2" line and .601 sq in for the 7/8" line.
Based on the pictures and what he wrote, I'm gonna guess he cut the flanges off the pipe.
Right, I dont really like that idea
his reduces even more if he is keeping the stock pipe ,even though it is thin wall. + the AN is also restrive imo
maybe im fussy