IMHO, your outfitter is attempting to give you good advise. For a once in a lifetime elk hunt, especially one where someone else is footing the bill, I'd recommend you upgrade your weaponry to the task at hand.
Rifle:
**Get a good fitting rifle (use someone that really knows the ins and outs on this part), preferrably a bolt action repeater [if that fits your handicap](but a repeater of some sort, especially if you don't want the outfitter/guide backing up your shot).
**Add a Pachmayer or Limbsaver -heavy dutyrecoil pad even if the stock has to be shortened to make the overall fit come out right.
**Go with a heavier rifle or have (removable) weight added to a lighter onesince it sounds (polaris) that you are riding to your ambush spots and likely will shoot from a rest.
**With your muzzlebrake I'd recommend using one that can be screwed off and a protective thread protector cap put on.
[Wear hearing protection with EVERY SINGLE SHOT with a muzzlebrake.]
[Make sure your outfitter will allow muzzlebrake use.]
Scope:
3x9 sighted to zero at 200 yards --- Western distances will potentially seem mightly long compared to most backEast deer hunting.
Cartridge:
30-06with 168 Barnes TSX, heavier if you go with lead core bullets (180gr.)
With this set up the emphasis was on the rifle so you don't have tocompromise on cartridge. Your guide will have absolutley NO second thoughts with this set up and recoil should be quite light. For a poll of favorite elk rifles used preferred by those who actually take elk, check out this thread:
http://www.huntingnet.com/forum/tm.aspx?m=1459421&mpage=18
You'll be right in the running!
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If this rig leaves you a little gun shy, then consider the same rifle set up ina 308 and 150 to 165 gr. Barnes TSX. It is a real close 30-06 equivalent, though seemingly not particularly popular in elk hunting circles. In any case, the fit of the rifle, the mag recoil pad, the weight of the rifle, combined with the muzzlebrake will pretty well reduce this rifle to about nothing significant on recoil.
Note: Not all bullets are created equal, especially when one starts "lightening things up." Barnes is your friend on this exercise.