I don't believe barrel length has anything to do with powder choice . . . the powder that pushes a bullet fastest out of a 26" barrel will push it fastest out of an 18.7" or 16.1" barrel.
Your best bet may well be to start with IMR4064 since you have it. If you want, you can try a box of Federal Gold Medal Match in 168 or 175gr first to get an accuracy reference point. I believe the charges are 42.8gr and 41.8gr respectively if using Federal cases. Since both use Sierra SMKs and IMR 4064, you can go from there to try and beat them.
A lot of folks swear by Varget, personally I swear at it - never could get a load that was better than the two I mentioned. But that was probably just me and my Czech rifle
IMR 4350 is fine for 30-06 but a bit too slow for 308. Don't know the others you mentioned for 30 cal.
FYI for my 308 I use IMR4064 + Sierra 168 SMKs and N140 + Lapua 167gr Scenars, both in Lapua brass. The Scenars hit steel for me out to 940yds which was as far as my scope would allow elevation + holdover. For 30-06 I recently switched from SMKs to the new 175gr TMKs (much better BC) + IMR 4451 (less temperature sensitive) with great success out to 1200yds, and will be trying the 175gr TMKs in 308 next to see if I can't consolidate bullet inventory. You might want to try the 175 TMKs + IMR4064 or the new Enduron powder IMR4166 (less temp sensitive than IMR4064).
But it all depends on how you want to use the loads. What do you plan to do?