727 yoke wont come out. What's the trick ?

Popping the cap will not achieve anything, except to give you access to lube the splines, and something to have to repair later.
A)I cant imagine the splines to be rusted to the yoke.
B)I can imagine that the yoke is too far into the tail and has run up onto the unsplined area of the output shaft. A couple of taps with a metal hammer should knock it back off.
C)its also possible that the yoke at one time welded itself to the bushing, broke loose and now is stuck in the grooved section.
In A and B above, the yoke and shaft can be moved back and forth, to the extent of the M/S endplay,maybe as much as .030
In C above,the yoke will move only a few thousands
If I had to guess, Id go B. Good luck.

If youre thinking of converting that motorhome tranny to street duty, you will be wanting to make other changes as well.I cant imagine the VB will be right, and Im sure the governor will be wrong, and the input better match your chosen convertor, and the TC better fit your crank.