The easiest way to remove the pistons is to get them nearly out simply by using the brake pedal. Remove the pads, and force them out BEFORE undoing the hydraulics. Then remove the caliper and try your best to get them the rest of the way. Air can be dangerous, grease is better. There is little room to pry on the Girling pistons in the 122 caliper.
The pistons have to be perfect - no rust pits. And there will more than likely be rust pits. I've used John Farrel's stainless pistons in Volvos, Ferraris, Aston Martins, Jags. Great parts, great prices. And they never corrode. You may have to source new pistons, and that's one place I'd recommend.
When assembling don't use brake fluid. Use assembly fluid, which is a silicone oil usually. EIS makes a good one. It's not hydroscopic, so it doesn't promote rust.
DOT 4 is good, and DOT 3 is fine too. If you can boil DOT 3 when driving, let me know. Corvette ZO6's use DOT 3 from the factory. Should work in a 122.
I used to use conversion fittings on the calipers from the bubble flare to inverted, and then use a common pre-made brake line bent to fit. 12 inch I think. Cheap.