Re: 351 to 302 length rods in a 351
in the combo you suggest a 302 rod would be out of its league of power handling, but the rod length is a good choice.
For a circuit car, or any other car being held at sustained high revs, longer rods can be an advantage.
They give a better rod stroke ratio, which in effect givees greater dwell at top dead centre, hence more torque and smoother high rpm.
The other advantage is of geometry. Because the rod is longer the angle to the bore is reduced at 90 degrees before and 90 degrees after top and bottom dead centres. (and everywhere in between)
The net effect of all of the above is that the pistons exerts less lateral force on the cylinder wall and reduced risk of punching holes in blocks.
The longest rod combo is around 6.250" with a custom piston, and would be advisable to resize the crank down to 2.10 inches and use chevy big end bearings. This also reduces speed across the bearing face and big end failure risk.
Hope this helps