The pump is just an At3k pump that is fitted into the front handle, and the tubing is routed into the tank. A ball pump will be even better, as the stock 4B pressures aren't that high, and the ball pump will fill up in 1 or 2 pumps.
To route tubing into the 4B, slowly drill in a hole. I used the grinding bit on a dremel to cut out most of the material, then took a knife to the area while holding the tank up so that the plastic fell out of the tank rather than into the tank. If you do get plastic into the tank, you can flush it out with water, and then just set it aside to dry for a while.
Then shove in 1/4x1/8 vinyl tubing and superglue it on. Add seal and strength by slowly layering on goop, as a large amount of goop on at one time takes forever to cure. I went with 3 layers, the first 2 4 hours apart, and the 3rd one 24 hours after.
Hack up your shell so the pump will fit and then add scrap foam to take up space before hotgluing it all together.
And now you can pretend to have a SM1.5k (provided you pick up a turret somewhere):
