PRM, I had a 23" std. chamber 338/06, read LOTS of articles on the round by Finn Aaggard, Steve Timm, and others. The consensus showed IMR4320 to do very well.
Mine shot 2909 w/200s, 2790 w/215s and 2670 w/225s. IMR4320 shot top speeds and accuracy for me and I don't think much more could be gained and stuck with it, 1/2 MOA was norm in the Hart tube- a hvy sporter. RL17 might be just right, but I'd be 4350 would be better with the 250 and up class. If you want to try another powder and perhaps pick up a tad speed if needed, you might try 4320.
A 21" tube might cost you 30-50 fps IMO over the 23 I had which is moot. FWIW, I used WW brass, partial sized, std. primers. My throat was long, done for 250s, so my OAL was out there, maybe helping case capacity.
Your goal should be in reach safely IMO.
