Both my sill covers have small crack from the underside and about an inch upwards about 10-12 inches in from the rear end, since both mine are cracked in the exakt same place I'm wondering if this is a common issue?

I'm going to drill a hole at the end of the crack and then weld them back up. Will probably weld up the jack holes to achieve a smooth look on the sill covers while I'm at it.

Carefully check your longitudinals under the rocker panels to be sure they're not rusting.

They are very solid (I have the hole car to pieces for restoration)

Can you post a picture? I don't recall ever seeing a crack there ...

Pic 1

Pic 2

Pic 3

Aha, yes, i have seen this before.

I would mount them first, make sure they align right with the rear of the fender and then weld them up. Shouldn't be a problem.

Standard vibrational cracking of rocker panels. They crack where the long ends.

Might be a sign that you're missing the metal tabs inside your fender that are supposed to keep the lower portion of the rear fender from vibrating.