I am kind of curious. I have a Mossberg 500 with a 20 inch barrel and 7 round mag, and the mag is only slightly shorter than the barrel (less than an inch). How long is your mag compared to the 18 inch barrel?
I also have a regular 870 express (meaning long barrel and standard magazine)and was buying additional choke tubes for it. It seemed to me there was a larger selection of chokes tubes for the Remington (from Remington and other manufacturers) than other brands of shotguns and the prices were slightly less. I am not sure whether this would be a consideraions for you or not.
If you already have a customizer in mind, you may want to quiz him with the options and his recommendations.
I kind of think a 20 inch barrel would be shorter enuff for turkey hunting, but it is your project. Good luck with it.