Is there a way to make an old internal hd into an external HD. I was thinking if you could create an assembly of sorts to plug an old internal into and then hook it (by means of the asssembly) to an actual operating computer -through let's say USB - that would help me out tremendously as I have about thirty old internal HD.s from old computers that took a dump.

You could get a hard drive enclosure
This should give you an idea what they look like and work but you can choose where to get one
External IDE or SATA Drive enclosure with USB 2.0 and Firewire interface from Addonics