Will front bucket seats from a '69 Ford Cougar fit in a '69 Chevelle?

Brackets are easy enough to make. 1/8" thick bar stock, a vise and a good size hammer and you're on your way.

I'd imagine they need to go up, which is good as you can't really get seats lower.
My buddy's Cougar had very low seats according to my memory from 40 years ago.

My first car was a '68 Malibu that I put a pair of '71 Cougar buckets in place of the bench seat. If I remember right, I had to drill a couple of holes for each one but I kept the Cougar seat tracks.

If the OE seat tracks are still on those buckets leave them on and make brackets to adapt them to the Chevelle's floor.That way you still have the seat adjusters on each seat.

those Jegs seats should have a universal seat mount I would see if the couger seats will bolt to those tracks, if not then you will most likely have to make your own adapters.

@dsjarmy You might have to modifiy the bracket to fit those seat but might not, the brackets Jegs uses are the same one's scat uses and are a universal type. I would do some measuring and see. Might get lucky might not. Either way all it cost you is time. If the brackets on the Jegs seats fit the car you could make strips as said above to mount those seats to the brakets you have already if nothing else.