Welcome Aboard Pappy.
On a '76 you should have an electric shift, so it's neither a 400 or 800. It's an electric shift. The model number on the drive never helps me find anything: Engine HP and the tag on the block is what get's me along.
Your engine should be an inline 4 153 Chevy. So the block tag will be something like 990223A11 MT (there are about 4 alternatives). Otherwise go by HP on the drive.
The 400 and 800 designations come in after '77 for hydro-mechanical and mechanical drives based on engine size (400 for the smaller engines, 800 for the larger ones).