I can't speak much about the J2 as I couldn't find one when purchasing headers but I like the 2.25 exit on it rather than the 2.5 on the NGM which I do run now. I'm also running stock 2.25 downpipes as I believe the 2.5 exit and 2.5 downpipes are just too big for the 3.8. I believe they both run 1.75 inch runners which I think is perfect unlike the 2.0 ARK header runners.
Circle inlet on runner that connects to exhaust head. Ports on Head is oval.
Can't speak on quality.
2.5 inch Exit
Great quality/welds. Going on 2 years no issues.
Well made collector.