I have a Tikka T3 "varmint" model in .308 (it has a 24" barrel, somewhat thicker than the standard model). I assume the receiver, bolt, and magazine are the same as the "standard" and "lite" models.

I am not able to reach the lands with most bullets unless I seat them too long to fit in the magazine. I would like to fix this.

Despite being a .308, it's a long-action rifle. The magazine has a spacer at the rear. I suspect the only difference between the 30-06 magazine and the .308 magazine is that spacer (which is integral, not removable). I'm thinking I could buy a 30-06 magazine, remove the spring and follower, make my own spacer about 0.100" smaller than the .308 spacer (thus allowing cartridges to be that much longer), and use the spring and follower from the .308 magazine.

Does anyone know if the 30-06 magazine will fit the .308 rifle? My magazine measures about 3.535" long at the top. The spacer (basically the thickness of the rear wall) measures about 0.608". Here's a pic.

Never played with the Tikka but I have removed the same type blokc from the back of a MKX 308 to allow me to use longer cases and bullets. I would bet you could swap the magazine or at least remove the block.

I think that the only difference in the Tikka actions and magazines is where the bolt stop and block are located. So yes I think that will work.

LWD

The bolt stop is located in the same position on all T3's but the bolt stop for the LA is different from the one for the SA.
To allow the bolt enough travel to pick up a longer .308 cartridge you would need to either replace the SA bolt stop with a LA one or machine the SA bolt stop to allow longer bolt travel.
IIRC you can buy a new bolt stop package from Tikka for ~$45 and it contains both a SA and a LA stop.
The .30-06 mag will fit just fine and can be used without modification for a long loaded .308.