You will still have a restriction at the control block, i don't know what they are designed to handle, I'd suspect a bit more than the standard. The other concern I'd have is the strength of the shaft from the engine to the pump. Install a larger pump you will increase the load. The engine may also not have the HP to drive the larger pump AND drive at the same time. There are a few factors to consider, I'm sure it would work, but you just need to watch a few things.
I'd never dare say it CAN'T be done, as i know the abilities of some of the people around here and they could do it. If it works as hoped is another question all together.
If you decide to give it a go, make sure you take LOTS of pictures for us, it will make for an interesting project indeed. I'd suspect the first Hi-Flow 743 around these parts
Hmmm.....
I think if I was going to build hi-flow onto a 743, I can imagine mounting a pump to the PTO off where the fuel pump is, like a tractor set-up.
I would then pumb into a manifold with an adjustable flow divider, relief valve, and electrically-activated control valve. I would mount a button on top of the left stick, click-on, click-off.
Then I would relace the tubelines with larger tubes or even hoses, and tee into the line going to the female or bottom coupler from the diverter, with a check valve.
With all that adjustable flow and relief, then I would see what I intend to run and do some math to get close, then dial it in under operating conditions. I cannot imagine a 743 being able to handle more than an addtional 6-8 gpm, but that really depends on the operating pressure of the attachment.
If you could hook a pump to the spot where they mount pumps on tractors(I'm blind and dumb here, now) I think you could solve the room problem with that approach. Return plumbing would have to tee into someplace that would for sure hit the filter and cooler, but teeing into the line going to the coupler would by-pass the control valve and eliminate the back-up there. You can't have high-flow in both directions unless you run it all through the main control valve.
So then with this set-up I guess I would want a way to disable the hi-flow button in reverse.
I imagine sitting in the seat, shifting the lever for normal flow, then hitting the button for high. Maybe an indicator light in the dash so you know when it's on, and don't inadvertently over-flow a standard attachment.
Very custom. Practically, probably more of a conversation piece, but I'd like to know what you're wanting to run with a high-flow 743? Sounds like fun, whatever it is.