When you say it is popping, is it actually making a popping sound or is it really just slipping and then grabbing again?
When it slips, don't let it do it for long or you will overheat the clutchpack and be rebuilding for certain.
Verify that the gear selection valve on the transmission is all the way in the forward position when you shift the control into forward just prior to hitting the point when engine throttle begins. If it is not going all the way, that is likely the culprit. If it is all the way forward, you will not accomplish anything by adjusting the linkage.
Are you checking the fluid level immediately after shutting off the engine? Fluid from the cooler will flow back to the trans pretty quickly and give you an artificially high reading. Low fluid will cause slip under load.
Finally, you might change the fluid, when you do, look at how much clutch material you suck out. If there is a lot of material, probably plan on rebuilding.
Enjoy the restoration process, if you stop having fun, take a break
Not sure how long you have been looking/searching the threads, there are a lot of great examples, (this is where I have to link to mine
Good luck, and welcome to the board! Be sure to post some pictures when you get her done.