In my experience the G-62 tolerates large propellers better then the GT-80. The biggest prop I have been able to run comfortably on my own GT-80 is a Fiala 26x10, at just over 6K, and a cut down Fiala 28x8. The latter is cut down about an inch or so, but it rips like crazy. Rpms are about the same as the stock 26x10.
I have tried a Menz 26x8 on my own G-62 many years ago, and I believe the rpm was about 5300 or so. But even then the engine ran fine, and for a Pup, that do not need that much power, it should still be ok I guess.
I would get the 26x8, and if itīs too much for the engine you could cut down the tips until you reach a spot where the engine runs ok. My experience is, that even taking an inch of the diameter makes a real difference to the prop load.
You could read through this thread.
http://www.rcuniverse.com/forum/rc-s...t-11-a-29.html in which there is some discussion about the G-62 and large propellers for WWI era birds.