Ford C-4 trans question

I just took my 55 sunliner on its maiden voyage with the 351 and C-4,the trans is a junkyard purchase and it took a while to shift into third and did good on the first and second time it went into third but now it will slip a little before going into third but does not slip while in third. Is there anything I can do to it or just drive it a while and see how it acts as the trans has probably sat a few years,before I installed the trans I installed new seals and filter and new fluid. :confused:

I used type F and the old fluid was not burned so i figured it should be a decent trans,if it does not straighten out I will send it to the trans shop for a rebuild. I only paid 60 dollars for it and they pulled it so I wont be out much if its junk,it shifts into second good and reverse is goot too so hopefully there will not be much wrong with it and I installed a new modulator valve too.