While I don't believe he'll reach 390", I think most everyone is underestimating this bull. He does have good beam length and decent tine length and I'd put him somewhere in between the OP's guess and everyone elses. He is easily over 350", and I'd guess a lot closer to 360". A definite shooter in my book.
My buddy killed a 340" bull with very similar beam length 4 years ago (56"), but his front half (brows and 3rds) were much shorter than this one. This bull doesn't have huge brows or swords, but he has good beam length, good 3rds and good 5ths.
I know of a gal that arrowed a 390" several years back, and that bull simply had it all: huge brows, long spiraling 3rds, long swords, 5ths, main beams and mass. The most perfect 6x6 I've ever seen.