If I recall correctly you can plumb the pressure side with the 1" barb, or the 3/4" barb. I used 1" tubing and the 1" barb. You can get a 1" barb for your return pipe at home depot to plumb it in. Alternatively, the threads are 3/4", and you can screw in a 3/4" to 1" fitting and then plumb normal schedule 40 or schedule 80 to the bulkhead. I'd recommend flexible because it is a lot quieter than when I had it plumbed with pipe. I'd also recommend using a union in the plumbing regardless of whether you go with flexible tubing or hard pvc.
Jack