Here is the challenge:
Yes a Milano clutch assembly will fit in your 84 GTV6. However, the e-bay item does not appear to be the full clutch assembly. The assembly includes the flywheel as well as the pressure plate, disk and throwout bearing.
You might be able to use your flywheel from the old assembly, but only if your car has the been "updated' with a single disk clutch previously. Of course you would want to have the flywheel surface refinished - not easy because of the shape. Then you need to have the assembly ballanced at a drive shaft shop (dynamic ballance due to the length of the assembly versus the diameter)
In the end you probably have a couple hundred $ in shop time to get the assembly to come together. And if your car has the original twin disk assembly, the flywheel may not fit on this pressure plate.
$500 is a big risk for me - maybe that is playing money for you - don't want to kill your fun
There are two other paths to consider:
1) new or good used clutch assembly. As long as it is complete either twin disk or single disk will fit. Don't swap the throw out bearing from one to the other - ask me how I know. I don't know the current availability or cost of new clutch assemblies, but I'm sure you can search that out quickly.
2) if the disk(s) in your assembly are worn, but the surfaces of the pressure plate and flywheel are good, you can reload either with new disk assemblies. Or even have new material riveted to the old disks. Local clutch and brake shops may be helpful - ask if they will re-line a clutch disk - or maybe start with Andy at performatech. I know I ran across someone on line who could give you just about anything from street to race kevlar.
Good luck.
mark