will 360 j heads fit on my 273

Yes, they'll bolt right on. Your compression will take a serious dive due to the larger chambers. With a 273 you'd need a pretty hairy cam to see any gains from the larger ports.

If you need heads and these are cheaper than rebuilding your current heads it may be a good deal. But if you were to slap them on an otherwise stock 273 and expect to go lot's faster, you may be disappointed.