The 4028815 is a 1978 360 999 transmission with lock-up converter. If the trans has the original converter with the weights on it, it is for the 360. Stock flexplate is all you need.
I will need this info in this thread, @RustyRatRod has the wisdom to all this so we do look a little dumb asking these questions
I have a 360 (I will get the date here soon) and a heavy stack (new build) non lock up 904 I will be matting together so we will be back to this thread some time soon.