Whatever you got should work since you can always stain it, it's such a large piece that grain run-out is not an issue really. At that point just choosing wood that has desirable properties for how you're going to work it and how you want it to endure afterward, and also picking for grain appearance. If you are going to carve by hand then something soft with grain that's conducive to carving is best, like sugar maple. For aesthetics, most types of mahogany would look pretty sweet and I've always liked the grain on it for guns like this. I usually hog it out as far as I can on a bandsaw, then use a profile sander like this to power carve the rest of the way, then finish by hand. This sander is kinda expensive but there's a harbor freight version that's like 50 bucks. Either way you do it, have fun!
Makita 9031 1-1/8" x 21" Belt Sander https://www.amazon.com/dp/B00004YOH7/ref=cm_sw_r_cp_api_glt_i_ZNMTHACJP96K6M7PV9HZ