A904: 1-2 shift happens early despite governor weight- any way to adjust this?

I'll be honest. I love manual gear boxes and I'm fairly clueless about slush pump transmissions. I have great faith in the honored Chrysler Torqueflight...so if I can't row my own gears, this is the transmission I would pick. Anyway, If this is an ignorant question, I beg your kindness in advance. :)

When getting my Dart back on the road a few years back with the Magnum swap, my budget demanded that the manual transmission transplant be put off for some future project so I did the best I could with the stock A904.

I had a local shop, Best Transmission in Springfield, MA do the rebuild/upgrade work. They have been around for more than half a century and plenty of old hotrodding knowledge is still on staff...good guys up there. I told them what my goals were (strong enough for a healthy small block, firmer shifts, higher rpm) and they helped me pick out some components. They were impressed how despite 140k miles behind a 318, the transmission was in nearly perfect condition with plenty of life left, had I not been looking to upgrade. No surprises there for a lot of you guys, I'd guess.

Anyway, I've been slowly chipping away at the tuning for the past few years and the car is running better all the time. What I'm finding is that the 1-2 shift is happening at about 4,400 RPM while the 2-3 shift is happening around 5,300 (about where I would want it, I suppose). Is there some means of adjusting one shift point but not the other? The transmission shifts firmly and kicks down predictably...I think I'm just giving up a huge amount of performance with that early 1-2 shift.

This is the setup:

Magnum 300 5.9 crate engine with the stock mild cam.
A904 with stock ratios for 1973/318 car
Upgraded clutches/bands, deep pan
Transgo TF2 shift kit
"5800 RPM" governor weight (subject to many variables, I know)
TCI Streetfighter/2600 RPM stall
4.10 final drive

Any suggestions are welcome.