SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Start over at the pump (keep the pump and the suction-side plumbing)

You'll never use all the water the pump can push out of the open pipe - at actual operating pressures, the flow will be much less

A 1.5" strainer will support the flow of a 1.5" line. I agree with boots, a 100 mesh screen is ideal for sprinkler systems.
As for assembly, I think you'll be suprised with the amt. of give you'll have once you make the cut. I would assemble the entire assembly prior to cutting to get your precise dimensions. FYI...the second shut off valve isn't necessary, I just like to have the shut off after the filter. Eliminating the second shut off will buy you some space also. It is, however, important that you position the pressure relief valve between your pump and the filter.

It doesn't matter as long as it's positioned before any device that could decrease flow while the pump is running. I.E. plugged filter, closed shutoff, or faulty sprinkler valve. My installs go...pump-spigot-relief valve-filter-shutoff valve. Spigot goes on top (same as yours) to make priming easier and before shutoff in case one wanted to use a garden hose off the spigot, they can close the shutoff and still be protected with the relief valve.
As for the relief valve...not a good idea. That relief valve is preset at 30psi. Most sprinkler systems run closer to 40psi operating pressure. So that valve would blow before your sprinklers even popped up. The adjustable relief valves I linked are more expensive because they are adjustable. I would look for one that is preset closer to 60-70psi. But, you have to be careful, because your system may be operating in this pressure range. Of course, the adjustable relief valve takes all the guess work out of it!