Autopilot Installation Questions

Just ordered a new Autopilot Digital with SC-60 cell and have some questions. I have a 24" horizontal run of 2"D PVC on the outlet of my Raypack gas heater which goes to the 3 way valve sending the filter/heater discharge to the pool. Have the following install questions:
1-How best to install the manifold? Since my heater discharge is 2"D and hard piped to the 3way outlet valve, how do I glue the manifold into the 10inch space that I just cut out (the pVC may not have enough give to spring open)
2-Would I be better to use 2"D unions on the straight run of pipe vice hard gluing the manifold in?
3-If I use unions, what type should I buy to prevent leakage since this section will be under as much as 35psi pressure.

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