TH400 kickdown and top speed problem M1008

Hello! My name is Luca I am 22 years old and I drive my CUCV M1008 since over 3 years.

3 weeks before I had to change the Injection Pump on my 6.2 Diesel. I upgraded to a 6.5 IP. Everything went well and the truck idles fine and has good power for an 6.2 Diesel. Also when I accelerate normal it shifts fine, no banging or high rpms shifts ... everything works like it should. But since a few days my truck has following problems: when I floor it and use the Kickdown it stays too long in 1. gear and shifts very hard after maybe 2 seconds in the 2nd gear. But then it does not upshift in 3rd gear. Doesn't matter how long I am flooring the gas pedal.
I also ran down a little hill and floored it so the transmission had to Kickdown. 1. to 2. gear was the same like on a flat road but the transmission shifted in 3. gear after maybe 6 seconds of holding the gas pedal to the floor. It seems like i can not reach the speed to get the transmission shift into next gear although I am on full throttle.
Another symptom is that my truck only runs 65mph (stock tires and gearing) before it ran easily about 75mph.
the things i have already checked:
-vacuumpump is good
-all vacuum lines are in good shape
-governor is not stuck and in good shape
-I hear a clicking noise when I put 12 Volt on the Kickdown solenoid so it should work?!
-also checked the vacuum regulator valve and it works like it should

Have you guys any idea how tho solve my truck´s Problem??

End to the week I am gonna change the transmission oil and filter (just changed tranny oil and filter 2500mls before) and i will see if there will be any suprises in the oil pan...

Thank you for every help from a CUCV brother over in Germany!

A wild guess from afar. I would adjust the vacuum regulator on the right side of the injection pump. Can you see the marks were it originally was? If so make sure it is on them marks. Do you have the top vacuum to the transmission and the bottom one to the vacuum pump? Also are you sure the vacuum pump is good? Hard to guess and diagnose from afar. If I can't get it set up I drop $20. in a friends hand that owns a transmission shop and he fixes it up in a couple minutes. Not sure that is an option in Germany. Good Luck.

Thank you for your reply. I have tried different positions of the regulator valve and there was no difference.. yes the top goes to the modulator at the transmission and the bottom one goes to the vacuum pump. I have replaced the head of the vacuum pump 2 months before and in my opinion the vacuum of the pump seems good. Poorly i have no tool to measure the vacuum the pump produces but it is still strong. Yeah actually I am in Hungary 800 mls away from home. And in 10 days I have to drive all the way back to Germany... So maybe I have to find a transmission shop here that has enough knowledge to fix it up