The simplest solution is to replace the 3x4 Graflex back with a 4x5 Graflex back not a 4x5 Graflok
I like being able to use the roll film holders and bag mags between my cameras but Efke has now gone leaving only Wephoto or the Ilford special order run and quarter plate FP4 + is twice the price of 4x5 here in UK.
For me, I just loaded up some film holders and made some exposures. I didn't need to find a 4x5 Graflex Back. For my first shots I just bought some 8x10 sheet film and cut five pieces of 3-1/4x4-1/4 and viola, I had t-max 400 to use the camera. Quite simple. If you cannot make a 3-1/4x4-1/4 camera work as is, you aren't trying very hard.
"She's always out making pictures, She's always out making scenes.
She's always out the window, When it comes to making Dreams.
It's all mixed up, It's all mixed up, It's all mixed up."