I just put an HDMR on a Manners T4 stock with a heavy varmint contour barrel. With LaRue's LT104 mount (which is a little higher up than the 807) the scope was WAAAAY too high to get any kind of effective cheek weld. I just switched to an American Defense Recon-SL mount, which is lower than the LT807, and the scope fits perfectly.
TL;DR, you should be good with the LT807 and the 34mm tubed HDMR. Try at your own risk though.
Hope that helps.