Pill Shooter,
I've given some thought to building a 35 RUM (actually a 35/338RUM) on a Rem700 action. Are you using the full-length 300RUM or slightly shorter 338RUM? I know of a 35/338RUM in Australia and the owner needs to seat deeply to allow cartridges to feed through the magazine, particularly with the 225gr Nosler BT. Have also heard the same regarding the 358STA and 250gr projectiles. My thoughts on barrel contour are along the lines of using the same contour used by Remington for its Safari Classic 375H&H model but extending it to 26". This was a much slimmer contour then used on the RemBDL in 375H&H. I personally do not like carry rifles much above 8.5lb (3.8kg) so that would be my top weight. I find I can carry that weight all day in steep country and it handles well for an off-hand shot when required. Add a decelerator pad and keep the original stock if its currently comfortable. Sounds like a great project.
Cheers.