HP limit with 1-5/8 headers

I run a NHRa stocker with a low compression 340. I ahave tried all the different types and tube sizes. I ran the 1 5/8 at first and tried the 17/8, the 15/8 was run for the torque they make over the 17/8. I have three sets of headers for my car and pick the right ones for the track and air. My 340 ran 10.83 in bracket mode ar Rockingham in Oct. 1.38 60ft and 6.80 in the 1/8. With the 360 spend the money for headers on a convertor and other needs first before the headers. The gain might be much for the money spent. The step headers that cost 1500.00 are worth about 10hp over your 15/8, the 17/8 are worth about 5hp and a set of custom stainless that costs 3000.00 are worth about 20hp over your 15/8.