"Buy the standard version and have the stock cut to fit you."
I started shooting my Remington Model 58 when I was 11 yrs old and weighed slightly less than 100 lbs. My dad chopped off the stock so it would fit me and just kept adding spacers to the stock as I grew to full size. I've been shooting that same gun for more than half a century now. Glad he didn't get me a kid's gun.
Enjoy your 870 !