
Internals:

Basic setup, with short pump stroke (about 2 1/2") and tank. For some odd reason, my OPV pops at 4 pumps, while I've heard of most others taking more than that. So I went ahead and plugged mine (I know, bad for warring), but I can add a different one later. I know McMaster OPVs are bad for letting out tons of air, but I'll slap one of those on, then just make sure to count how many pump strokes I can do without it engaging.
Ideally (as Draconis suggested, I believe), you could replace the pump with something of longer stroke/larger air output, and keep this OPV. It would up the RoF significantly.
Back to the internals. Here is the tube the missile sits on.

Cut it off at the end of the tube (LEAVE ROOM AT THE BACK, so as to not mess up the output or anything), and cut down the ridges between the tube and tank.

Wrap a couple times with e-tape. Take this following piece (about 1" of 1/2" CPVC, with 1/2" of 1/2" PVC), and stick it in a wye, CPVC facing the mouth of the beast. Then slide back over the e-tape, and use adhesive. You've now eliminated a bit of dead space, and made the wye attach perfectly.


Carve out the top of the blaster, for a hopper-loading slot. Easy to do, the ridges on the shell actually give a good guide for making your cuts. If you want to keep the missile holders that come with the blaster, carve a couple attachment slots in front of or behind the usual attachment point. You'll know it when you see it.

Wrap the end of the wye in etape, slide over a piece of 1 1/4" PVC, then slide on a coupler of that size. Cut a tiny chunk out, to center it on the wye and keep things lined up nicely.

Part 2 below.