I am looking at buying a 68 RR(?).  The dash VIN plate says RM21H8Gxxxxxx.  I see no stamp on the cowl (like my 70 Challenger) to corroborate.  However, there is a stamping on the radiator support that reads G8Rxxxxxx.  This seems to indicate a 383 2BBL built in Windsor.  The seller says he has the matching #s 383 4BBL and 4-speed, fender tag and build sheet to verify the St. Louis built car.  The doors have been dipped  clean.  I guess the radiator support might have been replaced due to a wreck.  How else can I determine whether or not this is a clone?  

If it is a 383 two barrel car which the tag on the rad indicates, then it is likely a cloan. No 383 road runner ever cam with a two barrel carb.
If the vendor has the correct fender tag then why was it not on the car. Piroting tags from other cars has become a regular problem, you might be wise to walk away on this one if your concern is to purchase a genuine Road Runner. However Cloans when done right  can be just as nice, and they are fetching similar money as the originals.
Years ago I purchased a 70 road runner which ironically was registered as a Satellite. You would never know to look at it with everything being in place. The nice thing about this was I could register the car with the insurance company as a Satellite [lower premiums and saved a great deal of money considering I was only 21 at the time.  

You might also check the numbers on the engine block. They should match the VIN # of the car. At least this will tell you if it is the motor the car was born with. The 68 should also have the motor type designation on the top front of the right (drivers side) valve cover. A 68 383 RR motor should have 383 HP and a letter designating the year (dont have the letter designating a 68 model off the top of my head) stamped into the flat spot on top of the block in that location. Hope this helps some.