Annoying "POP" in clutch pedal after new clutch kit?

[Got Smoke?]

Ok I pulled my transmission & transfer case out of my 79 K20 a couple of months ago to install a new clutch. I had my flywheel resurfaced and ordered a new clutch kit from South Bend Clutch with a diaphram(sp?) pressure plate. I installed a new clutch fork and put new bushings in the Z bar linkage, added the LONG clutch fork return spring (ordered both but couldn't make the little one work?), and also swapped out the large garage door style OEM spring for the new style pedal return spring.

Now I have a EXTREMELY annoying POP when releasing the clutch pedal about half way up. The problem comes and goes....sometimes it will do it time and time again, other times I may drive for 30-45mins before it does it again. Its a distinct enough pop though that if I'm not paying attention I can almost stall my truck when it happens.

An old mechanic friend told me there are different length throw out bearings for our old trucks, and that most likely I got a short one in my clutch kit and need a long one? He's usually right on the money on his stuff, but I've never heard this before? Anybody have any insight?