If you want to move it a lot, I'd consider attaching the end cabinets to the cart better (a couple of bolts near the bottom).
If you want your lid to close, you will want need to modify the lid. The lip of the end box, which goes over the edge of the cabinet, keeps the lid from closing. You could also build brackets on the side to hold the end boxes...a piece of 1/8" flat bar, with a flat bar spacer would let you hang the end box like it is designed, then use spacers and bolts at the bottom to make it plumb with the rest of the box. In fact, I'm pretty sure that is the way I would go: build a bracket of with flat bar material to attach the end box to the side of the cart. Attach the flat bar bracket to the side of the box with countersunk machine screws and put another piece of flat bar on the inside as a backer. paint it with rattle-can red and nobody will know any different.
You will likely want to attach the handles on the end box on the non-steering-caster end of the box.
Based on the casters on my 5-drawer cart, I'm pretty sure they will handle the weight you are talking about. With the wheels more to the center, it will tend to steer better, but you may have a problem with it being a little 'tippy.'
If you got those end boxes for just over $100, then you got a steal. I could only get them down to $150 for an end box who's keys were locked inside it. Those are definitely worth the price, even at $200. If I wasn't planning on building my own, custom tool box, I'd seriously be considering a couple more of those.
If you need to lock your tool storage, call HF customer service and order a set of 'keyed-alike' locks to replace the locks on your cart and cabinets. For a few bucks, you can stop fiddling with multiple keys to lock up. Of course, you could also enclose the bottom space with sheet metal and make a door for the front and get an extra lock for it too.
Post pics! I want to see this.
Kev
