For the mono send you should use a regular mono guitar-type cable, meaning tip and sleeve, no ring.
The return is a little harder, as the 7600 has no 1/4" out. Your custom cable should do it, as long as all the XLR pins are connected correclty. It needs to be wired:
XLR 1
1 – gnd (Sleeve)
2 – hot (Tip)
3 – gnd (Sleeve)
XLR 2
1 – gnd (Sleeve)
2 – hot (Ring)
3 – gnd (Sleeve)
You will probably need to boost the gain on the input and cut it on the output by the same amount.
