Im no expert but I've done a ridiculous amount of research in choosing headers for my build. The general consensus on primary size is where you want your power, smaller = low and mid range while larger = more top end. Port alignment and collector size/design are every bit as important as primary size. Just because you saw gains with larger primaries doesn't mean it is because they were larger, its the complete package that makes a difference.

Superchevy has an article on a header test using a smeding 383 and 4 different size headers. The 1 1/2" primary, while intended to just establish a base line for the test, supported well over 400 hp and ended providing the best overall power curve below 5000 rpm. The larger sizes didn't pull ahead until 4500+ and at the sacrifice of low end. Headers by Ed states that there is no point in having a primary size larger than your exhaust valve so if you have a typical svc with 1.5" valves, no need. Unless you're building a high rpm motor, bigger is not better.

If your talking vortec heads, the biggest problem is port alignment. Unlike standard sbc heads, vortec's have a raised port (approx 60% of the port is above the bolt centerline) so smaller primary headers tend to cover the top of the port. Bigger primaries while uncovering the whole port present sealing problems at the bottom of the flange.

I myself went with a set of flowtec 11508 flt headers (less than $150). Basically the same headers in superchevy's test, 1 1/2 primary and 2 1/2 collector. Replaced the wimpy 1/4" flanges with a set of 3/8" cnc cut flanges that I bought on ebay from Hells Gate Hotrods. He sells a set of sbc for $45 and made me a custom set for the vortec port alighnment for less than $100. I went a step further and bought a set of Meg's weld on merge collectors from cone engineering for $125. After finish assembly I sent them to a place called pro kote Indy in Indianapolis and had them professionally cleaned and ceramic coated for $220. Just got them back and they look awesome. For less than $600 I got ceramic coated, perfect fit headers with perfect port alignment and plenty of sealing surface.

Assembling motor now. Let you know how the do when I get it running.