Building a commuter bike from a early 2000 Giant ATX which had Alvio components.

I was using the Alvio crankset (IG chain only) with an old 7 speed cassette and worked well with Exage index 7 speed shifters.

Fitted a wheelset I had lying around with a Shimano 7-speed mega cassette and now the chain pulls and creaks on chainrings (i.e. the chain appears to be slightly to short between the rear cogs and front rings). The index on the Exage shift also doesn't seem to like the Mega cassette.

I would like to use the 7-speed mega cassette (or at least the wheel it came on) with FC-M440 crankset off a 9-speed.

Obviously I have compatibility probs - can I buy a new chain and fix prob or will these components never work well together? Would I be better off to change the cassette to the old one that worked.

Appreciate the advice.