IIRC, it's as simple as swapping out the .30-06 barrel for a .308 barrel, and possibly installing an adjustable gas plug if you're worried about over/under-gassing the system. Might also need a spacer block for the mag well (some people say the en-bloc clips don't have any trouble feeding the shorter rounds, others have issues).
Barrels can be had for around $200-$250, the spacer blocks are $20-30. Not sure about the adjustable gas plug though.
Far as finding a smith to do the work, I'd imagine that anyone with experience working on Garands could do the job for you.
My very first Garand was a Springfield Armory built on a GI receiver. It shot great but I can't say that it was better than some of the 30-06 rifles that I've had.
The Garand is a very fussy rifle that requires a good sling and consistent technique. Everything that you do in hold or sling use changes point of impact. I shot Garands in NRA 200 yard competition for a long time and while I love the rifles, I got fed up with the fussy nature of the rifles.
I'd suggest that you pick up an M1A in 308 instead. The cost will be about the same and you'll shoot better with it. The M1A has a shorter operating rod, it's less vulnerable to op rod damage, uses replaceable box magazines and it's a more consistent rifle.
Be warned, however. I've had National Match Garands, a full match, pedal to the medal M1A and none of them would shoot with a decent AR-15 A2 in 20" bull barrel from 200 to 600 yards.
Flash
.....................just dont want to wade thru the unsolicited advice!
Not Dallas but you might give Eric a call at National Match Armory in Burleson. They also have a website. Just a FYI some find his personality a little.....challenging.
Strange, that never happens here, must be an anomaly.