I did go to their website and there is a pic of the block stamp. Not very good, but I can see the ID suffix for assembly application and what looks like a partial vin, but there is a small c prefix that looks odd.

Underside pics for evidence of rust issues or panel replacement ?

Yeah a few obvious things wrong, which makes me wonder about the rest of it.

66' Impala SS-427 L-72
 

My older Brother had a white on white Bucket, SS-396 4-speed car he bought from the original owner back in 1972 I helped him do a frame on restoration with a paint job (my 3rd) in 1974. I am a big fan of the 66' Super Sport Impala and had a sizable collection of all NOS part to restore mine, but sold it all with the project car. One would think the seller would at least level out the front bumper so it doesn't look like someone jacked the center of it with a chain. Yes the air cleaner assembly is wrong for a 66' L-72 Impala. I had an early take off / original one in my collection of parts. You've got to love the lines of the 66' fastback Impala coupe, especially from the side. BIG GM cars with BIG engines, now them were the days
Bill