Schumacher headers 440 in 71 Duster

Tri-y works well and fit better than and full length big tube headers but yeah they're not a high rpm header but most big blocks peak well under 6,000 rpm and unless you take your car to the strip or dyno it it's probably way less than 500hp.
How much power is the tri-y making more than a set of cast iron manifolds? Is it really worth it?