Shorty headers vs stock manifolds

I have installed some performance improvements and thinking about the exhaust to replace stock manifolds with shorty headers. Will the added hp be noticeable (i.e. more than 10-15 horses) under this setup?

it is 73 302 engine with slightly enlarged exhaust head ports, weiand 2-plane intake and holley 4777 with mechanical secondaries, mild cam 220 duration (at .050 lift), .47 lift, late model pistons increasing the compression to low nines

I was rather looking for some specific hp improvements.. I have a set of JBA shorties from late 70s mustang that from the initial "they will fit somehow" turned out into a overall refab job for the one on the driver side. It has flange in the worst area and hits the steering box; pass side is OK. And I can't get new correct headers. I was wondering if it is worth to refab headers for the hp improvement. Whether it is 10-15 horses range or more like 40-50 horses range...

I know 4777 may be too large, still have original autolite J.I.C.