Marvic will not (cannot) alter backspacing, their wheels are sand cast and it would require all new molds. I have Marvic wheels, GT5 wheels here and GTS flares, but may be a while bfore I can find time to off all that up for measurement. Start with the distance from the flare edge to the hub and we can figure the rest out. FYI the Grp 4/GT5 wheels will require longer studs.
I do like the 10" GTS Campi on the GTS and you can always use a thin spacer to improve aesthetics.