How will installing a 18 inch rear mag From a 1979 XLCH where a 16 inch mag is on a 1981 XLH work out?

Assuming the same side profile, it will be a lot bigger. It will also throw off your speedometer by about 4MPH at 65 and change your ultimate gearing. If you can change sidewall profile, get the same circumference and then you will be OK.

It won't throw your speedometer off since that is run off the front wheel.