There was no magnesium wheel option for the C6, so you have C5 wheels.
The optional C5 magnesium wheels look like this:
Now, doing the offset math...
The 1984 wheels appear to be either 16x8.5"-32mm offset, or with Z51 pkg, 16x9.5"-38mm offset on the rear.
If the calculated dimensions below seem confusing, keep in mind a wheel's stated width is 1" less than actual. Stated width is measured where where tire bead seats, and does not include the lips. But, backspace and frontspace dimensions does include the lip thickness, which is 1/2" inside and out.
The C4 wheel specs work out to this:
16x8.5"-32mm = 6.01" backspace, 3.49" frontspace
16x9.5"-38mm = 6.75" backspace, 3.75" frontspace
The C5 wheels are:
17x8.5"-58mm = 7.03" backspace. 2.47" frontspace
18x9.5"-65mm = 7.81" backspace, 2.69" frontspace
So yes, those C5 wheels will be tucked in about 1" more than the stock`84 wheels. If the additional backspace doesn't present a suspension clearance issue (I don't know), they may bolt on and work as-is...But if you want them pushed out to fit closer with the fender edge (to fit about the same as the stock wheels), you will want a 1" thick (or 25mm) spacer.
With spacers that thick, they will come with their own studs, and have holes and thin nuts to attach it to the stock studs. You might have to trim the stock lug studs to do this, or replace them with shorter ones, but I believe 1" are thick enough to avoid that problem.
And yes you want them to be hubcentric, and for that, you need to know the centerbore size, which is 70.3mm.
Here's an example link of what you're looking for (note these are generally sold in pairs, you will want them all around, so 2 pairs):
2 1" inch Hubcentric 5x4 75 Wheel Spacers Adapters 70 5mm Chevy GMC Pontiac | eBay
I'm making no claim of the quality of those, they are very cheap, and personally, this is not a part I'd go cheap on. If you want top quality spacers, look at H&R, but you'll find them to be quite expensive in comparison, $250-300 for a set of 4. I think H&R part#5085704 are the ones that will fit this application.
Hope that helps
