I'm the sort of guy who replaces everything I can whilst I'm at it. In this case I would probably replace the HP line at least unless it is known to be recent.
Just a thought to spend more of your money!
Going back to attaching the hoses. I've not done it this way, but if you're using the short HP hose, you could fit it/both before installing the box. This might be a bit awkward, but if you find a way to hold them up out the way while you bolt the box in, it might save some agro. This of course means it could get a bit messy when you disconnect the HP hose off the pump and reconnect the new hose. Use Flare wrenches if possible.