904 with very hard 1-2 shift

Do you have a functioning KD system?
Do you have a proper 3.91 governor set-up?
cuz if it has 3.91gears with a 2.76 governor, the increased governor pressure, caused by the increased driveshaft rpm, is gonna throw everything outta whack. I always fix this first.
Then;
If that lever down at the trans, is tied back; it will jack the throttle pressure ALL THE TIME, instead of just at large throttle openings. You can untie it, readjust it with a longer tether, then go for a spin, but stay out of the Secondaries. If it shifts better, readjust your KD linkage. Do not drive it without at least some throttle pressure , else it is possible, at WOT to cook the friction materials due to slippage.
If you take the pan off to readjust the line pressure; here's what I did;
I drilled and tapped the case, in line with the adjuster, for a small pipe plug.
When the engine is shut off, the oil level will be high enough to pizz out that hole, so, I only jack the driver's side up. Now I can adjust any time I want and trust me; you'll NEVER get it just right the first time. Out on the road, I just drive up on a curb and skootch in there.

BTW
Line pressure combines with Throttle Pressure, to fight governor pressure, which commands the automatic upshift speeds.
If I have to, and I usually do, I adjust the Governor Pressure FIRST, cuz if it has 3.91s with a 2.76 governor, the increased governor pressure is gonna thro everything outta whack.
In my limited street experience, I prefer LESS line Pressure and MORE throttle Pressure to achieve the same Combined pressure, which I adjust up or down, depending on the engines power level and the load (car weight and rear gear ratio.)
The reason for all that tuning, is to prevent clutch slippage when the Secondaries open, yet have a comfortable driving experience, in auto-shift mode.
And,
after I get all that dialed in, then I fool around with the 1-2 shift valve to get a nice lifted-foot automatic downshift back into first. I like it when I come into a turn, slowing down to not be too hot, and then BAM!, it downshifts all by itself, and I rocket out of the turn. For me, that is just so much fun, when it happens automatically.

A&A has the governor kits, there may be other companies IDK.