904 throttle pressure questions

Working with a 904 that I was told came out of a 71 duster V8 318. I had it rebuilt by a local here that does trans for lots of people and Ive never heard a complaint. Hes a racer and has been doing this for almost 20 years.
So i tell him Im going to be running it with a 408 and give him all the specs for motor, convertor and gears. I also tell him the convertor will be a 3800 stall. I let him know I plan on doing some racing so I want to have a manual valve body and a good shift kit so I can feel a harsh shift. He completed it about 3+ years ago but life has gotten in the way and I am finally getting this thing on the road and the bugs worked out.
Im not happy enough with the shift from 1st to 2nd or to drive. So I called him and he told me I needed to check throttle pressure. I put a gauge on it and I see I have around 90-95 PSI when in gear at the port. This is my first time checking this or making any adjustments to a trans in the pressure area.
So here are my questions, I want a harder shift, the builder is more worried about the comfortability of the shift. he says I could go to about 105 PSI. I ask if I could go higher if i still didnt have the shift I want and all he will say is how uncomfortable its going to be.
So how high can I turn the pressure up before it causes issues.
I need to build a bracket and some type of adjuster to make these adjustments since I dont have any kick down linkage on the car.
I noticed tonight When I checked the pressure that it did not changed with engine speed so Im guessing that once I make any adjustment it will run there at all times?
Appreciate any information and schooling you can pass my way,
Rod