This should probably be in the Tactical column but here is the deal on the M1a. The best bases would be Smith Enterprises and Sadlac. http://www.smithenterprise.com/products03.html
The down side to scoping an M1a is cheekweld sucks unless you add a cheek piece like the Karsten http://www.snipercentral.com/karsten.htm
Then of course you need some rings badger ordnance or Leupold Mk4 at least some warne rings. As for choice of scope you need one that is really tough and the Leu Mk 4 or Super Sniper 10x would be a good choice or a fixed 6x or 4x Leupold. The gas system of the M1a really punishes a scope so most wont live through it. If however you do not want to go the cheek piece route you might consider getting the scout type rail form Springfield Armory and mounting an aimpoint. Or mounting an aimpoint on which ever base you select because cheek weld in not important with an aimpoint.