If I was to build a 300yd 6BR for myself, I would start with a barrel in the 24" to 26" range, likely in the range of heavy varmint contour, with probably a 1-12 twist and shoot 80gr flatbase bullets. I think you may even be able to reach 600 with a 1-12 and 80gr FB's. The reason I would use the 80gr FB's, is at 300yds, the wind is going to start to have more of an effect than at shorter ranges and the heavier bullet with higher BC, compared to the 60-75gr bullets, will hold up better in the wind. And they should also tend to yaw a bit less in early flight that the longer 90-115gr boat-tails and VLD's, which should help a little with short range accuracy.
But, where you are shooting prone, you really need to base the barrel length/contour on what will balance well and feel right for you. Having a short and stiff barrel is useless if you can't hold it steady. (bear with me here, I don't shoot highpower and I am not the most knowledgeable on the rules) And since you will be shooting with irons, I believe anyway, extra barrel length will help as it will add sight radius, unless you employ a bloop tube which can compensate for that a bit.
I don't shoot prone, and my 6BR is not yet finished and will be a 1-8 for heavies at 600yds, but I do have a .222 I built for 300yd shooting, and I went with a mid-range twist rate (1-9") to shoot medium weight bullets. It does better at 100-300yds than the 1-7 or 1-8 twist .223's that a couple of my friends shoot, and they are using 75gr and heavier Boat tail and vld bullets.
I have found in my recent testing of lighter FB bullets, that my groups at 100-200 yds are better, most of the time, with 50-60gr FB bullets than with the longer 60-75gr boat tail's. But, it does still shoot the BT's very well, just not quite as tight as the FB's at shorter ranges. I think this should also hold true for the 6BR at 300yds as well.
My 6-.284 (1-10 twist) shoots far better at 100-300yds with flat-base bullets than it does boat-tails.
I would try the 80gr Fowler's (http://www.6mmbr.com/catalog/item/1308478/779476.htm) or the 80gr Bergers, which are listed on the Berger site under Varmint bullets (http://www.6mmbr.com/catalog/item/1308478/839547.htm) and use a 1-12 twist barrel. And if you want to try the lighter fare, the 1-12 throated for 80's will be far better suited for the task than a 1-8" throated for 105-115's.
Good luck, hope it works out for you.
Kenny