Sounds like a grand plan. If you have a roller / surcingle use that, otherwise run the long-lines through stirrups. Or, being as you're tall you can just have them straight from the bit. Try with them just on the halter for the first five minutes, so it you do find yourself stepping on them while you're getting the hang, you won't hurt him. Get in an enclosed area, so if all else fails, you can just let go (my lot all spend a considerable amount of time on turnout with leadropes attached so they know darn well what to do if they step on a rope at any time).
Establish your verbal cues first - even if he's done it a bunch before - get the two of you used to your accent first. Just lead him on a lead rope and check that you've got 'Walk on' 'Trot' and 'Whoa' all sorted. Then with the long lines, do exactly the same routine from further back, and then further back until you're right behind. I like to spend a lot of time on circles, just as if we were lunging, but with contact on two reins. It's just like riding (dressage) from the ground. You should feel a huge difference when you get on right after long-lining, if you're trying to ride on contact too, because you've just been able to watch the effect of your hands directly on the pony from the ground. That's about it really - I tend to be walking 6ft or so behind pony at the start, and then move them out on a circle until I'm at the end of the lunge lines.
Oh - don't try the flying change too soon - you have to be awfully feckin quick - mind you, with your legs, I bet you could do it too ! (I know - PNH single-liners can do a flying change while keeping the person's legs still, but on two lines, you have to do an instantaneous leap to the center of the other circle 20m away as they cross the center line !)