according to my Hollander's 39th edition, the '66 convertible had a frame that was shared with no other vehicle or year. as Big Dave pointed out, it will be a boxed frame.
when I had to replace the tranny cross-member on my '67 convertible in the early '90s (x-member was rusting out), it dropped into pockets that had been welded on the frame. however I had to use a x-member from a hardtop which was wider. I know this because I actually pulled mine from an open frame in the junkyard with no body shell on it. My recollection is that that frame was not boxed and simply had holes drilled in the frame for bolting the x-member down. All the curves in the x-member were in the correct place, the x-member was simply an inch or two wider on each end. I clamped it flat in a vise, trimmed the ends, drilled new holes in the x-member ends and dropped it in.
I mention all this as to give you a reference point. Unfortunately, as Dave pointed out, the only real way to be sure is to look for the hidden VIN stamped on the frame. according to a thread I found on chevytalk forum (chevytalk.org/fusionbb/showtopic.php?tid/343316), my hidden VIN is located on the driverside frame top. some people even said they did body off inspection and found no stamping whatsoever. take it for what it's worth,let us know if you find the VIN frame stamp.
if you need some visual references, try searching on Google for the following string "1966 chevrolet passenger vehicle hidden vin" and click on the images category after getting search results
Last edited by 65_&_67_Impala; 09-14-2020 at 02:25 PM.