Okay, I am going on the assumption that you have the following.
1978 400M with 2wd. The distributor should be set at 12 degrees BTDC.
1979 400M with 4wd should be at 10 degrees BTDC.
Seems to be a wide variance, but it should all get you into the right ballpark.
Best thing is to buy yourself a Haynes or Chilton's repair manual or look for the emission sticker on the front panel by the radiator. I had thought all of that was easy to find on the INTERNET, but I pulled up so much trash, it was hard to cipher through the mess. My repair manual is for the 80 on up trucks, but I am fairly certain it is similar. Most of the 80's stuff has you set it to 0 degrees and then the ECM does the hard work for itself.
