i do something similar with some of my varmint rifles. i'd shitcan the 20" and go with 24" in standard bull (.750" from the block forward and 1" behind...) - why not? you're shooting off a bipod or rest - why not get a few more FPS? also i dig the A2 stock with trap door lead wedge. this adds 3 lbs or easily removeable lead to the rifle and it balances, if there is such a thing when they get that heavy, better than a unit that is nose heavy.
if you're using a harris, pick up the 9-13 model. the shorter units won't let you use thirty round mags easily. when you leave the mag in, the unit will rest on the mag instead of the grip... also, i've experienced really good versatility and great accuracy with a 1/9 twist - it'll stabilize 40gr ballistic tips all the way to 65gr+ units... it doesn't shoot VLD's however... if you plan on not getting much above 55gr/62gr then the 1/12 are little more popular.
two cents.
septic tank