.308 is 7.62mm. If the dealer knew that it had been rebarreled in .308 and he still told you to buy 8mm milsurp, he was placing you in great danger.
7.62 x 51 NATO and .308 winchester are effectively the same caliber. There are some small differences due to US and European specs.
8mm is a different caliber entirely, about .32 caliber.
Make sure that the caliber of the weapon matches the caliber of the ammunition before you shoot it.