There are several methods of lengthening a stock; if you want it to look good, I recommend they be done by a stockfitter, a gunsmith who specializes in working with the customer to come up with the right stock dimensions for the shooter. The top end option is a whole new custom piece of wood. This would cost more than your 500 is worth, quite honestly not the best option if you want to use that gun. The best solution would be to install an adjustable LOP buttplate like this;
https://stockpositioning.com/produc...t-plate-assembly-auto-for-auto-shotguns-black or this;
https://stockpositioning.com/produc...ustable-lop-pad-plate-black-stock-positioning. SPS is the go to for these, and it's best to call them (number is in the link) and talk to Joe, he will get you set up right. From your last post, this might be a good addition also;
https://stockpositioning.com/products/4-25-rectangle-heavy-duty-comb-adjuster-step-insert , if you decide to stay with the 500. (or on any gun shotgun you decide on.) Another shop I recommend is
https://tronspace.com/ , and again best to call, and his name is also Joe. These two shops are the best in the biz, they will get you fit correctly.
With your size, you will probably find that getting a fitting done on any longarms you plan on keeping is a good idea. You can't shoot well if the gun doesn't fit you, and as you've found out, most guns won't fit you well as is.