99camaro5.7

OK so i just bought my 99 camaro z28 a few weeks ago and every now and then i have a problem with 5th and 6th. At random even when i slowly let out the clutch or try to slam it in gear it doesn't make a difference it still happens at complete random. The problem is it grinds and will not go into gear (that is 5th and 6th only no problems with other gears) I can eventually get it into gear after a few trys. I was told the transmission mount is worn out, i haven't checked it myself but could this be the cause? I also believe the rear main seal is blown and while everything is out im going to put a clutch kit in with a little heavier duty clutch, could this help it also? I just don't want to have to worry every time i get to 5th and 6th about if its going to grind and miss or if its going to go into gear. Please any suggestions or any ideas on what it could be or what to do to fix it!? thank you