I'd get lots of different 12AX7 types and try them all out.....you'll find one you like best eventually, and then you'll have extras in case it fails. Get a NOS 6AN8 for the phase inverter, and a couple new JJ KT88's and you're good to go there. As far as caps...well stock is probably better for guitar, but it all depends on what you're looking to get from the sound. All the mods I mentioned in past years are meant for bass, and I personally can't stand the sound of the Sunn tube amps for guitar except for clean tones, so I never really considered their use for guitar and what mods are appropriate. You could go either way...the bigger caps on the first two cap sections would give you bigger bass...but whether you want that or not I don't know. My choice for an amp for guitar would be a Vox AC30, tweed and blackface Fenders, vintage Marshalls and HiWatts, Silvertones, Supros, etc. Hope that helps?
Greg