Me too...........my junk went 11.80`s-90`s on a little tire at an altitude track but he`ll find it.......Is you car runnin yet?

Let me ad a small story to show you the importance of proper convertor sizing. Here's my Sons combo

70 Duster (3100 pounds)
4.10 gears
28-10 slicks with caltracks
.040 over 360 engine
11.1 flat-top pistons
stock crank
ported Edelbrock heads
ported Victor intake
750 alcohol carb
904 with 5600 stall convertor

His best run with this combo was in the 10.50's

A transmission line broke and fried the transmission so we installed a fully prepped John Cope Racing transmission and one of two convertors I bought from a Moparts member. Next time out the car slowed to a 11.20 with a just freshened convertor that stalled at 4500 rpm. I called and talked to John and he said the difference between a 904 and a 727 is minimal with the PROPER convertor. We pulled the transmission again and installed the other convertor. Next time out it stalled at 5400 and ran 10.50's again and has since run in the 10.40's. These engines LOVE high stall convertors.

NO-ONE be afraid to say what your thinking...I'm asking for help and won't get offended.
I think you hit the nail on the head.
I attempted on the 3rd pass to bring the RPM's up against the converter having a bad feeling my old 750 would stumble.
I looked down at the tach and in trying to tighten everything up knowing that I should be able to get to maybe 3K I noticed it was trying to spin like in the burnout box. I dropped it back down to an idle and left and sure enough it stumbled.
The converter is 36 years old...but not used at all for probably 25 of those years.
Like I said before I used a lot of old stuff I had laying around. I only spent $600.00 on this engine less the heads, headers and distributor. I sent this converter out before using it because of a cracked pump snout...the bolt holes were b!tched up where it bolted to the flex plate also. I got it back in an all new housing for 180.00...I have an idea its nowhere near a 3500 any more. Maybe that explains why I never noticed any slippage with the 3.23's as I thought it may slip some...
To be honest I doubt it gets changed out unless he talks me into 3.91 gears all the time rather than 3.23's. Would going to say 4-4200 be a problem on the street with a 3.91 gear?

Absolutely not and I've seen higher with a good cooler mounted up front. To bad you weren't closer as I would loan you a convertor to make that sucker fly. Surely someone close to you could let you test one

Oh and the 5600 stall that was in Matt's 904 was my transmission builders street convertor but again he knew how to keep one cool

That car is so cool the ET doesn't matter; ) Chances are you're going to bracket race it rather than run heads up anyway. A mid-low 12 second car is a great bracket car. I completely understand wanting to go as fast as possible with the given combo, but your times are nothing to sneeze at. Good luck finding the ET you are after.