i have a series d (4x5 ) and have been known to chase my kids around the yard with it
why not sell the camera you have and get one that requires no modifications ?
you might get what $$ you need for the super ?
or use a roll back so you don't need to deal with sheet film and holders ?
( i have a roll back taped to the back of my series d and use it all the time )
modifying a 3x4 to 4x5 doesn't seem too difficult. you need to remove the old back
and probably shim the new one so it is the right distance to focus correctly.
maybe ... someone who has done this can tell you the right size shims to use
and all you need is a back, and a screw driver and some lathe ?
good luck with your fun project !