Distributor Spring Setup - SBC mechanical timing

Hey everyone. New to the forum, saying hi to everyone and have an advance question on timing and springs.

So first, my setup...
SBC400, stock block
Mild Erson cam (282/290), lash set to 0.25"
3500-3800 converter
TH350 trans auto
MSD Streetfire Distributor, just purchased this year
Edelbrock 750, rebuilt this winter, stock jets/setup
Aluminum heads (not sure on brand/setup off the top of my head)
Highrise intake with aluminum carb spacer
Longtube headers
WBO2

Timing: -20 @ 1000rpm, -34 @ 3200rpm. Vacuum advance is manifold vac, doesnt matter right now on exact timing, question is about mechanical.

So I've been working on this setup for 3 years, my first SBC. Been getting better and better. Been running great, good power this season, the most I've seen coming from this thing all year or ever for that matter. A problem I've ALWAYS had is the amount of idle drop going from Park to Drive/Reverse It's usually about a 200-300rpm drop, which sucks because I keep the idle high in order to compensate for this. The converter is the right size for the cam, so I know that's not the issue. Also have an issue of everytime backing up, she wants to die. Makes it very hard backing up into the garage, needs a lot of gas to keep it alive.

I bought the spring kit for the dizzy to try to retime to the mechanical and done a TON of reading everywhere. Today was the first time playing around with the springs. From what I've read, the dizzys stock springs are more medium, so I swapped the stockers with a heavy and a medium from the kit. I checked timing, and stupid me didnt pull the vac so my readings were incorrect, but was showing about -23. Kicking myself for not disconnecting everything and getting the right reading. I didnt check the full timing curve, went for a quick spin. Would been incorrect readings anyways. FML.

The idle was lower in park, and when putting the car into D, only dropped about 100rpm. Also sounded totally different, more lopey. When taking for a quick spin (not flooring it), she was very hesitant. And she just didn't sound that great. But may have just been different because there was less timing?? I'm guessing that there was no mechanical timing being added to initial?

So what do you guys think... does that combo sound about right? And should I retime for initial to be at -20 with the medium/heavy spring combo and make sure the curve is there to -34 @ 3000?

I appreciate the responses and help!

I'm more inclined to question an issue in the trans. If it's losing that much rpm that you have to feed a lot of throttle in one direction more than the other, it may be the linkage inside the trans is applying a band or clutch pack at the wrong time. How is the trans shifted? Has the linkage been abused, beat on?

Check out Denny's post / questions. If that doesn't help, consider having the trans linkage investigated.

Updated the original post to reflect the change... my notes show I had at .025 for the valves, not .022, sorry. Cam is E110822.

Trans shifts great, have small leak at the main seal since I sadly didnt replace when I replaced the converter. Not bad, and it's full. Shifts great though, no issues going into gears either. I doubled checked all the linkage underneath, had someone else shift through the gears while I was under the car, looks fine. Nothing binding or grinding.

Distributor is a brand new MSD Streetfire. Not sure what else you're looking for on that.

So I tried again with the medium and heavy springs. I reset initial timing to -18 (doing it the right way this time), did some adjustments on the carb screws using a vac gauge (about 12in) and reset the idle to 950 in park. When drop to D/R, idles goes to 900 and purrs like a kitten. Timing gets up to about -33 @ 2800rpm. Only time theres not a smooth increase is coming off idle to about 1150. But rest is there. Drove a bunch today (no WOT) and seemed very clean and smooth. Backed up no problem at all.