If it was converted to .308 with a insert bushing, I wouldn't shoot it. If it was converted by the replacement of the barrel with a barrel chambered in .308 then you are good to go. That's about all I know about them.
...Further, if it had been converted from .30-'06 to .308 with a Navy sleeve (that "bushing" thing), you really can't "un-convert" it.
Although you probably can remove a Navy sleeve by carefully jamming a tapered round file into it, and twisting and pulling, I believe that you would find that the .30-'06 chamber is essentially ruined by the imprint of the jammed-in sleeve.
Although I have no personal experience, firing a so-converted Garand, I am led to believe by the comments of others who have done so that the combination is not as accurate as the original configuration. The culprit, I've been told, is the excessive length and oversize diameter of the sleeve's freebore, between case mouth and barrel rifling.
The magazine-and-clip system works just as well with one cartridge as with the other.
I have three in my kit, each with one .308 cartridge.
The instruction I was given (verbally) with them was to insert both sleeve and .308 cartridge into a .30-'06 chamber, close the bolt, and fire the shot.
That is supposed to be sufficient to seat the sleeve and secure it in place.
For obvious reasons, I've never tried it. To me, the sleeves are "last ditch" devices, assuming a .30-'06 drought and the availability of more .308 cartridges.