http://www.youtube.com/watch?v=rL7YLuhHWsc
Start by drilling a 1/4" hole in your tank (don't worry about random plastic shards that get into your tank, we'll deal with those later).
Then solvent-weld in a section of 1/8" ID, 1/4" OD flexible PVC tubing. I used Oatey's Red ABS/PVC/CPVC cement.
Next throw a 1/2" to 1" PVC slip bushing on there.
Minimize your shell if you want. Mine started off minimized already. If you don't minimize your shell then you will have to do some extra steps and cutting to get everything to fit nicely.
Drill two holes in the left side of your shell, a 1/4" one near the top for the tubing to your tank, and a hole in the handle for your pump. The pump I used has an L-shaped head with circular face, so I could drill a 1" hole and be perfectly fine. If you have a pump with a weird face then you will have to get your dremel out and carve out the hole until the pump fits snugly.
You will also have to cut half-circles in the front handle on both sides of the shell so the pump tube will fit nicely in there.
Now mount your pump in there. If you made the holes nicely, the shell will take almost all the stress of the pumping motion and you will only need to fill in the gaps to prevent pump wobble. You may also have to cut parts off of your pump so that the shell will close. Depending on the pump you have this might be very easy to do or extremely risky. Shave off small amounts at a time to make sure you don't cut into the pump head and thus introduce a new air path for all the air of your pumping to escape to.
While we wait for our adhesives to cure, reinforce the trigger.
I cut out two pieces from 1/8" PVC sheet to reinforce the sides of the trigger and solvent-welded them on. Next I filled in the curved piece of the trigger with some epoxy-putty.
Even at absurd (read: totally banned) pressures when testing in the lab, I have not had this trigger break. Depending on how good you were at machining the reinforcing plastic sheet, you may or may not have to widen the horizontal gap in you shell that the trigger sits in.
Now cut off the OPRV from the stock pump and shave down the head slightly, and shave out 3/4" SCH40 PVC until the two fit inside nicely.
Grab your solvent-weld and attach the OPRV stub into a stub of 3/4" PVC.
By this time, the solvent-weld on you tank from the earlier steps should have set and be well on it's way to curing. Cut the pump tube off of your tank. Take a 3/4" spade bit and destroy the check valve in the back of your tank. Also dremel out what remains of the pump tube so that 3/4" PVC will fit nicely inside. Now that you have a gaping hole in the back of your tank, flush it with water from the front to get rid of all those plastic shards and shavings that have made it's way in there.
Finally, solvent weld the stub of OPRV and 3/4" PVC into the back of your tank.
