[GMCPaul]

I'd guess the clutch disc was installed backwards, never done this but rather sure I've heard if done that's the issue caused, otherwise I'd think of condition of flywheel and pressure plate were they heavily glazed? or did you surface them prior to install.....
I asked dad about this and he stated if it's backwards you normally can't get it into gear and if started in gear it tries taking off as it's already engaged ( also causes grinding if running trying to engage with truck running), he suggested possibly the glazed parts, incorrect throwout bearing or improper adjustment not letting the clutch fully out.
Paul Jr @ GMCPauls