1970 C2500 NP 435 transmission problem

[nrapozal79]

Hey everyone and thanks for looking I got this 1970 GMC c2500 4 speed. I believe it's has the np435, I bought the truck and it needed a motor I put a new rebuilt 350 in it and got it running great. But my problem is when the transmission gets hot it doesn't want to shift, I know first and reverse are not synced but to get it into reverse or first it requires a lot of force and makes a horrible loud grinding noise any ideas as to why this may be happening? it has fluid in the transmission, when it's cold the transmission goes into first and reverse smooth very easy and no noise only hot it shows that problem any ideas what the problem may be? thank you for looking