the cost for rear conversions depend on what you can fabricate and what you need to buy.
For me when I need new drums then it'll be the turning point that i'll do a rear disc conversion. I'm figuring about 300 when all is said and done, right now I'm collecting random parts from here & there. If you happen to get lucky '96 vans had disc brakes as an option and it's a complete bolt in deal.
jack up both sides,take both drums off and do one side at a time.you can run to the other side if you get lost.get a drum brake tool it will help with the spings.not as hard as you think they are.