Your engine assembler should determine the appropriate length for the pushrods in the course of fulfilling his responsibility. There is a procedure that is normally followed for accurately calculating length and it comes into play, especially when using roller rockers. Many factors affect the length, including the amount of material removed from the block/head components in the process of truing the decks, the thickness of the head gasket, the depth of the pushrod cup in the specific lifters used, and the base circle dimension of the camshaft. In my experience, factory pushrods for roller cam motors are typically shorter than those for flat tappet motors. One thing that you should verify is that the pushrods are hardened for use with guideplates. If you combine guideplates and unhardened pushrods, there is the likelihood that the pushrods will shed metal into the motor in the area that they come into contact. The fact that they are made from chrome moly material doesn't guarantee that they are hardened sufficiently. If the valve spring pressure and RPM are held at reasonable levels, there should be no real need to increase the wall thickness or the O.D. of the pushrod. For reference, you might wish to do a search for Manton Pushrods and read through the introduction to their catalog. I'm not suggesting that you will need to buy really expensive pushrods for this application but getting the correct length and hardness will help avoid future problems.