[JLMounce]

Honestly, if you want discs on the rear, I'd just go ahead and buy the kit you're looking at, instead of trying to put something together very cheaply just to see if you will like them. That last part being the issue. You're not going to "feel a difference" with a basic disc setup in the rear of the car.

The front brakes do the vast majority of the actual braking on these cars. The weight up front and the dive dynamics mean the rear wheels do relatively little. In fact as you start putting big powerful brakes on the back of these cars, managing brake lock-up becomes somewhat difficult.

The advantage of disc brakes on the rear don't really have much to do with outright stopping performance. Especially if the tire package isn't changing. The advantages show up in less unsprung weight, better heat dissipation for more repeatability and in my opinion, better looks.

Your car isn't going to feel much different or stop any better during a one time panic stop than it will with your rear drums.

If you are after the look, or know that how you want to drive the car will require better repeatability in the braking system, then you want the disc brakes back there and there's really no reason to try a set on first before going with the setup you ultimately want. If you're committed to disc brakes for the reasons above, save the time and money on your test setup and go straight to the setup you want. You'll be money and time ahead.