I've searched throughout the site and haven't found the info I'm looking
for. So now I'm turning to the C3VR Brain Trust for the answer, please...

On our '78 some (lunatic) PO removed the bolt that attaches the seat belt
to the floor. I'm not talking about the 'inner' mount, by the transmission
tunnel--the plastic 'sleeves' and receiver are still mounted in place there
with the correct torx bolt, fortunately.

In this case I'm talking about the location on the outer side of the seats--
by the door sill, in other words. I did find a post on here that pointed
towards Dr. Rebuild as a source for original hardware--and if I have to, I'll
definitely buy from them.

However, since we're not keeping the car original, I'm wondering if I can
find a suitable replacement bolt (good quality, Grade 8, stainless steel) at
my local hardware store. Problem is, I have no idea what size to look for.

Does anybody know offhand? I think I saw a mention someplace (I read so
much I can't honestly remember at this point) that this is a tapered bolt,
with no threading on the topmost portion...but I could be completely
wrong about that.

Any help would be greatly appreciated--even if it's only to point me in
the direction of Dr. Rebuild after all!

I would not reccommend using any other type fastener for the seatbelts, in any position. These bolts are specificly for seat belt anchoring. They do have a "shoulder" on the top 1/2" or so, to allow the anchor to pivot if needed.
I believe any seatbelt bolt will work, whether a torx type, or a regular hex-head. The inner bolts on mine are torx, but the outer shoulder belt(on the "halo") are hex-headed.
Why not take a trip to your local salvage yard, and get a few there, for little $$$?
Some auto-parts stores have these bolts, also.

Up to 1977, the bolts were 7/16"- 20 threads. After that, they are metric
Adams' Apple38894.7913888889

Joel's got it covered It is pretty much a corporate bolt. I use to install them by the hundreds a day. 

Since we live in an urban area, salvage yards are (unfortunately) quite far
away...

auto parts stores are a different story, though. Plenty of those around. As
well as a Chevy dealer.

So it sounds like if I ask for "metric GM seat belt bolts" I should be able to
find these--??

Thanks, guys, for the fast help. I'm going searching for them in the next
day or so...

I would think that any GM dealership should be able to supply these bolts.

Thanks--I'll be off to our local Bow Tie boutique ASAP!

I appreciate the help...