1. No need to remove the heads
2. You can get a fitting that has spark plu threads on one end and an air connection on the other compressed air holds up the valve
3.Buy a spring height checker....its a micrometer like deal to measure installed height. Summit should have them.
4.You will need a spring compresser that uses the rocker arm stud kinda like a prybar.
5. Inspect the retainers for wear but there shouldnt be any....no need to buy new retainers and locks
6. Its an easy thing to do common sense should guide you through if you have any mechanical aptitude.