When building a dry fire hydrant system the design of the suction head matters. About one-third of our work involves inspection, testing, and repairing/replacing dry fire hydrants. Often times we are asked to check suction heads that "don't work" and what we end up finding is an under-performing design. The Schedule 40 PVC 90-degree elbow suction head is a very popular design. If choosing this design then the best practice is to use a 90-degree elbow that has the suction fitting built right in (threaded is how most are done). When folks try to use a straight head or try to make their own configuration that often just leads to a quicker path to system failure. The more fittings and adaptors needed only increase the physical stress on the system allowing more opportunity for air leaks to occur.
The bottom line is the best practice is just to buy the 90-degree head from the beginning. If your current crazy configuration system needs replaced - just buy the right head - don't try to build it in Jimmy's garage because he has some left over fittings from a job two years ago.