At wits end......

Is this happening at WOT at only higher RPM's (like 4k or 5k) or even at lower RPM's and WOT, like at 2k to 3k?

Stock Carter replacement type fuel pump is spec'd at 25 GPH, which is around 250-275 HP..... might be fuel limited, regardless of the 3/8" line. Do you have the actual fuel pump model?