First of all, do not grind the lash caps. They need to be perfect so they will contact the roller tip and transfer the lift to the keepers//retainers.
I always run lash caps if I have a soild roller cam. No matter if you run them or not, the last purchase should be the pushrods.
The following are things that can change your required length:
Head gasket thickness
Block decked
Heads milled
Valve seats too high or ground too low
Base circle on the cam
Good luck, Dave