If you're keeping the machines to use... or even if you're thinking of just putting them on the shelf to display, I really can't think of any good excuse to downgrade a Plus to a 512k. Anything a 512k can do a Plus can do better... the only known point of incompatibility (disk related problems with copy-protected software) you'll have on the 512k if you retain the Plus' double-sided drive. (The double-sided drives don't use the PWM signal to control rotational speed, which broke some particularly evil copy-protection schemes.) A hybrid made out of your best case parts and the 512k motherboard won't be an unmolested "numbers matching" collector's piece anyway, so why bother?

I'm sure other people will have contrasting opinions, of course.