As it happens, for my double decker build I wanted to use 3/4" EMT for the side rails and 1" for the gantry and Z rails. So I made a side roller specifically for “C” rails and “J” gantry tubes. It’s part of a larger random collection of pieces for the double-decker here: https://www.thingiverse.com/thing:3709811
Specifically the part you need is: C_Roller_With_J_Gantry: allows “mix-and-match” rails where legs and side rails are 3/4" EMT conduit while gantry and Z-axis is made from 25.4 mm tube.
I’m not quite sure if the corner pieces need to be adjusted. The Z gap between the side rails at the corners determines the Z gap and must be consistent with the gantry rails, but with differing diameters it’s less obvious what the gap should be.