Help! Hooker Super Comp Headers

i have a 72 duster 340 stroker will hooker super comps fit my car these are 1 3/4 headers the engine builder recommended getting bigger headers i bought hooker competition headers a few months back but there 1 5/8 so he said the dyno will pull different numbers with bigger headers.
should i get bigger headers or stick with my 1 5/8 headers??
thanks
nick