There was no way to open up the ball on the shorties to anything close to 3" - the Mac prochamber necks down to 2.5" at the junction for a reason. Even if I could, I was limited by the firewall/floor pan and the JW Ultrabell - the Prochamber routing on the passenger side is less than ideal. Just not gonna happen - believe me, I looked and stared long and hard at it before going this route.
Sorry, I don't have a part number for these headers, I bought them used from Bruce Hemminger on here and they came with a surprising amount of dents and dings, not to mention a good amount of surface rust inside. I got them for $275, and by the time you add everything in (K member, spring perches, maybe a tubular trans mount), and with me doing all the work (thankfully I own a TIG and have a good assortment of metal cutting/grinding tools - bandsaw, die grinder, burrs, benchtop beltsander, even made a tool to knock dents out of the inside of the tubes, etc) including blasting and Cerakoting the headers at home, it's still going to cast a cool grand by the time I'm done. Fortunately, I'm down to the final work and I've got a bunch of Cerakote swatches coming to decide on a color. They should look great when I'm done (I hope).
In the grand scheme of things, I didn't mind going this route because I knew I'd be doing a lot of cutting and welding to make it exactly how I want it, and I'd hate to do that to $1,000+ headers. Heck, to minimize weld intrusion on the inside of the tubes, I'm using .035 MIG wire as TIG filler rod. It's interesting to say the least.
And yes, I think the Whipple is at it's limit. So that's a factor too. I expect boost to drop (along with IATs), so that might contribute to some gain - even blower HP consumption should go down. There's a lot of factors at play in my case.
'84 LTD LX - 9.83 at 140.09. Whippled 365 SBF with a glide and 3.08 gears. Driven to and from the track 60 miles without even changing tire pressure.