I'm getting ready to install an 847 cam in a 94 engine and need some help with the spring height.

I have:

gm 847 cam
cm's 612 springs.
pro magnum 1.6 non self aligning RRs
comp r lifters
comp hi tech 1 piece 7.2 pushrods
gm guideplates

the heads are stock and will stay on the engine.

What are the steps I should go through to find and set the correct height? (for this setup I mean)

Also, where can I buy shims?
Thanks!