I have a L579 and an AV SAKO the new style SAKO mounts are all too tall for the old SAKO action's unless you have a 56mm obj.
The best set up I have seen is the Burris dual dovetail bases with burris dual dovetail signature rings with the inserts.they come in 1" and 30mm
p.s. I have a set of origional SAKO L579 rings if you are interested but they are low's so you would need to have a pencil barrel or a scope with an obj. smaller than 40mm
if you say Leupold makes a 1 piece mount, I have never seen one. I am a huge Leupold fan but every SAKO mount that they make that I have seen has not fit well. that is why I suggested the Burris dual dovetail mount (With Leupold Scope)
According to the Leupold website they make one. But after more research, it does sound like fit will be a problem. I spoke with Ron at Warne yesterday, and he said the same even for their mounts on that action, I need to caliper.
My next prject is going to be a 700 plug and play!