1949 Chevy coupe

I have a '49 Chevy coupe with a strait six 235 and a 3 on the tree trans. I replaced my clutch, fork and fly wheel. My third gear is still popping out. My tranny seems to be good. I was gonna switch it to a floor shifter to see if it helps.

 

pull the trans and repair it. A different shifter will not fix the problem

 

2X for me

 

your shifter forks are worn, drain the gease, pull off the side cover, turn the forks 180 degrees and replace cover and grease; should work like new.
The forks get worn on one side, by flipping them 180 you get a "new" side to use.
...I'd try this first, easy to do and no $

 

You may also have a worn 2nd/3rd synchronizer assembly as well as other worn parts. That is often the cause of 'popping out of gear' too. Linkage and fork wear can be a contributor, but are not the only source of the problem, as others above have suggested.