It is not the most glamorous but when I had the core replaced on mine decades ago, the shop cut and opened a small flap in the inner fender. Somehow, they knew the perfect spot to make the cut.

After replacing the core and hoses, they then bent the flap back into place and secured it with a few small screws and coated it all with sealant. I have since reopened the flap to replace the core a second time. I re-secured the flap, sealed it and coated the entire wheel well with an original-looking asphalt coating as specified in '68 Pontiac literature. It is only noticeable if someone looks for it in the well and sees a slight outline.

Of course, the rest of the fun is on the inside, under the dash.