I am building a single shot rifle in 7.62 x 53R (basically a 7.62 x 54R with .308 grove dia.) I ordered a 7.62 x 54R reamer with a 30 cal. pilot from Manson. The reamer has a very long tappered throat and appears to match the C.I.P. drawing. With this throat, there is no way bullets can be seated anywhere near the lands. I don't know if this will be an issue or not but was wondering what others have experinced.

It wil perform as any other long throated rifle does; not optimal accuracy.
They do this so you can shoot .310 ammo out of a .308 bore.
If you want best accuracy you need to copy a real 308 throat.

DPCD is right about the throat. I built a heavy target rifle in 7.62x54R on a BAT action with Krieger barrel and had the reamer made to fit Lapua cases and .308 diameter bullets. The rifle was extremely accurate and the cartridge was not only easy to tune but achieved very impressive velocities without pressure signs.

I have the pattern stock mostly finished and was able to shoot a couple of test groups today. I think it will be okay.