Personally I would reconsider using it. Its tough to judge value, but if yours is in 85% condition and all original, it would most likely be worth considerably more than the link given by Hammer. The linked too gun is not original (reblued and has a cutts compensator). While interesting the cutts was not standard and detracts from collector value.
Again, depending on condition and year of manufacture and the right buyer your gun may go as high as $2000 or more. If its in poor shape it may be worth no more than $300 or less. The guns are fairly rare and heavily used and modified, thus leaving few good original models.
Mossberg makes an inexpensive 410ga 505 pump, which is actually kind of a neat shotgun. This would make a simple replacement for the 42. It is nice to hunt with old guns, but it is down right sickening when accidents happen, this would be compounded by sentimental value.
Some say it is a hunting gift from your Grandpa, you could also look at as a financial gift. EIther way, aren't you glad you asked!