So, I have had all the flack for wanting to fit a 602 in place of a 425 on a '63 Fourgonnette van AND retaining drum brakes. Read CATS Citroen page for cutting exhaust branches, mounting brackets and turning the whole thing around. Seems possible, BUT I don't understand the general system of the exhaust.
By retaining the original brake master cylinder and clutch pedal, the cross box exit exhaust pipe gets in the way, and the reason why the CATS Citroen note says swap it all aroind, but would it be simpler just to cut the exit branch off, and put it on the other side and patch the hole?
Also On the 425, the pipes join BEFORE they enter the cross box. On the 602, each cylinder exhaust enters the box on either side and exit once 'silenced' at the same diameter. Could they also be joined before the cross box to reduce the congestion around the brake master cylinder?
Looked at the racing pages, but that didn't help.

if there had been an easy way then Jeroen would have found it,

Look for "gutted crossbox" as there was a thread about it

STP did a build your own kit supose you could reverse it,

a T joint in a twin cylinder engine is less than ideal a 2in to 1 is possible if you can bend the pipes - keep the lengths and diameter as close as possible to original

messing about with the exhaust can leave the engine gutless the cross box is more of an expansion box to the tuned length of the primary pipes as it is a quietening device.