Poor idle and off-idle performance after distributor swap

Starting a new thread, as this started in a vacuum advance thread.

Here's the details: I have a freshly rebuilt smog 360 with a XE 262 hyd cam, performer RPM intake, eddy 1403 500cfm electric choke carb. It still has low compression pistons, should be 8.5:1 or less. It had a stock, 318 distributor on it, with a Pertronix Ignitor 1. The car ran fine. Idle was a little rough, but I attributed it to the cam. Car was reliable with pump gas and felt plenty peppy to me. 11.5 MPG, and it would do 6.8 second 0-60 just mashing the gas from a stop, no launch. Timing specs were 12deg initial, 25 mech. 20 vac can.

I noticed a sharp decline in fuel economy and rough running. After investigation, turned out the vacuum can had broke, old diaphragm probably rotted. I started a thread looking for a new can, and during that process I decided to swap distributors. I had a distributor halifaxhops curved for me that was an electronic; this is a points wiring harness car so I swapped the Pertronix into the new distributor and installed.

The new distributor has 20degrees mechanical and 15degrees vac. In order to swap in the Pertronix, I had to take the points breaker plate and swap it into the new distributor. I also had to drill a hole in the breaker plate for the curved-style arm on the vac can. It had the hole for a straight-style vac arm.

So that brings me to the current issue. The idle quality is poor, in order for it to idle right I have to turn the air screws out to like 4-5 turns, and the unburned fuel smell burns the eyes. And the transition off idle is rough, sputters a bit and sometimes even backfires. Mashing the gas, there is a slight hesitation and then it goes very well. The rest of the rev range, everything past initial throttle opening, feels excellent. 0-60 even increased to 6.4, even with the hesitation. The problem is most apparent when climbing my steep driveway with just a tiny bit of gas, runs very rough, surges and backfires. When idling in drive holding with the brake, the idle quality deteriorates the longer I sit there in gear.

I am trying to set idle using mixture screws and timing, I started with the transfer slots exposed a little more than square, and the air screws at 2.0 turns. It absolutely idled like crap, very erratic and timing would jump all over because it was way, way too lean. Now, it idles ok but the mixture screws are crazy rich. Pulling timing back to 12 did not help.

So that's the dilemma. Ran great, swapped distributors, now it runs like crap. My thought process:

1) Is just the swap from a 318 distributor to a custom-curve with less mechanical enough to cause this thing to need radical carburetor adjustments? Again, it ran reliably before.

2) Is the Edelbrock 1403, 500cfm not enough carb for my combo? Am I chasing my tail trying to tune this thing? Again, it worked before... with a different curve.

3) Did I mess up the distributor somehow swapping top plates? I was very careful drilling the new arm hole for the vacuum can, but it is possible I was a degree or two off. This should only affect vac advance though, right? My problem is of course showing up as I tune with can unhooked.

Other details: Plugs are autolite 3926. They were very clean, looked lean on the porcelain and sooty around the thread ring. Fresh fuel from a few days ago. New gas tank, nothing in the fuel filter. Carb recently rebuilt by a specialty carb guy. Ran great before the distributor swap.