To 400 yards the animal won’t know the difference between 7mm08 or 308.  Even for the shooter it’s hard to tell the difference, other than a few extra clicks on the scope.  If you’re using holdover tics on the scope, whichever better matches the tics is what I’d use.  For Kentucky windage with both cartridges I zero at 300, hold a hand width low at 100 & 200, a hand width high at 350 and two hand widths plus a finger width high at 400, with the 308 requiring two extra fingers at 400 for it’s extra drop.
Using your 7 mag bullets, the 7mm08 is what, only 200ish fps slower?  About 150 to 200 yards worth, depending on the bullet.  That’s pretty good performance.
I don’t see a benefit to anything heavier than 140s in the 7mm08 for ranges to 500 yards.   Same for 160s in the 7 mag and 165s in the 308.