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.