I'm not gonna bother with trying to coat the inside of my tanks.
If you do try it,keep in mind that POR15 is very picky when it comes to prep.It'll be real tough to properly prep inside the tank unless you do it before you weld the last cap on.Then you'll have the area of the closing weld open and it'll compromise the rest of the internal coating in the tank.When that happens,you'll have chunks of por clogging up you system.
Believe me I mulled over the same thing as you when I built mine and I finally decided to go without.
That being said,my tanks were severely overbuilt
.According to the engineer I talked to,they are good for around 1600psi!
__________________
'72 C-10 Proud owner of a million new and used pieces that used to be and will once again be(god willing) a testament to the term "they don't build em like that anymore. 
'80 LWB 2wd beater
'67 GMC long fleet (cab donor)
'66 long step (parked till it's turn comes around)
'65 short step (parts truck that will donate it's bed and possibly frame to the '66)
'06 Grand Prix wife's car (she hardly lets me touch it)
my pics