Re: Best .338Lapua custom barrel length
26" is quite adequate. My TRG42 with 26" barrel achieves 2675fps with 300gr SMK with 90.5gr Retumbo, which isn't a "stiff" load at all.
Buddy's custom has a 30" Bartlein, and he gets 2850fps from 94gr Retumbo.
Hard to really compare the two, as he has 4" more barrel and 4gr more powder, but not even 200fps difference.
Maybe 175fps are important to you, maybe not.
I think there are some fairly short 338's around here achieving some very respectful velocity.