Depending on your load,you are going to need 35 to 45 min of elevation to go from 100 to 1000yds. If you shim your bases and use rings like the Burris Sig w/inserts, you should be able to get the job done with the scope you have. If you haven't looked at them, consider the Leupold factory target turrents. They can be added for little money and work very well.
Other scopes to consider are the new Elite 3200 10X, the Tasco SS 10X42 if budget is limited (still very good scopes, own both). If you have more money, consider the IOR, Elite 4200, and VariX 3X9 Leupold Tactical.
If you have lots of money, consider the 16X LR Leupold, Nightforce and others. Not sure if you will get better scopes but they will be more money.
I don't have a clue what the difference is between them. I have the regular Vari-X III 4.5-14x40mm on my .30-378. Sorry I don't have an answer to your question. They sold that Sendero .308 anyway and I don't want to rush all over town trying to find one so I think I will go with a .300 Win Mag instead. Recoil is not problem for me, it doesn't effect me at all.
yes Leupold or Premier can upgarde your Leupold low turret with target turret but a 3.5x10 40 mm or a 4.5x14 seem not the good tool for 1000 yards even with target turret .
a used 3.5x10 Vari III is sale around 300$ used on EBAY and a 4.5x14 LR 500$ used as nib on Varminthunters adds so why spend time and money in shim the base and other home made improvment rather to get the right tool for 200$ more ?
Nighthawck in your first post you plan to built a thousand $$$$ 408 Cheyenne with a $$$$$ US OPTICS and now you want built a real cheap gun ???
A good scope is a must to shoot far and a target at 1000 yards or over is not a deer at 100 yards .