I've developed three different .243 Win loads for three customers of mine. One has a 26" barrel, another a 22", and last a 20". All of these loads were developed with the 105 A-Max, except customer 3, where I loaded for the 107 Matchking - which should be very similar.
The 26" Rem 700 SPS Varmint that I loaded for customer 1 shot best with:
Federal Gold Medal Match Large Rifle primer,
22" bbl #2
CCI LR primer,
and last, 20" w/ 107 SMK
Fed GMM LR primers,
not chrono'ed yet.
I wouldn't use magnum primers unless I were using heavy loads of Win 760, Win 780, H-380, or another ball powder... just my opinion.
Work up something trying RL19, RL22, or 4831 SC. I think 4064 will be too fast for your 26" bbl, but again, just a prediction, rifles can like weird things sometimes... Like a .30-06 I used to have that shot it's best groups using heavy bullets (180+) with RL-15... I never would've expected that!
I haven't tried H-1000, but hear great things about it.
If you are happy with the performance of IMR-4350, try H-4350 - from my experience with these, H-4350 is usually a tad bit slower burning, and seems to be less temp sensitive. A good friend of mine who hunts with a .243 almost all the time, swears by Win 780, however I've never tried it.
EDIT: I recently picked up a couple pounds of IMR 4007 SSC, which has a burn rate between 4320 and 4350, and I am getting stellar groups with it from a .300 WSM
22" and a 7mm-08 20" - and I'm predicting it will work excellent in .243 as well! Since powder is so hard to get right now, 4007 SSC is known so little, it'll probably be easy to find!