I understand. My '65 fastback was used as a drag race car for ten years...had a 351c in it during that lifetime. After buying it and figuring out its history, I made the same decision you are now making about your S-code. Unless you have a 390 already and money is an issue, my opinion is you'd be better off with a new 351 crate engine (I went with Ford Racing 302. I found this choice better fitting with my budget and ability to get parts). Note: 302/351w has growth potential not possible with the 390 and parts are a lot easier to come by. 351w block (or stroker block) can produce the same hp as a 390. Before deciding what route to take determine whether your toploader is 31 or 28 spline, what the tail shaft length is (short or long), what the rear axle is, 8 or 9", type and ratio, etc. Decide what clutch and pressure plate you wish to use and see if everything looks copesetic before starting. Sometimes what you now have is a big determinant it where you should go. Problem with picking an old engine is you have to scrounge up everything and sometimes it takes forever unless you've already got it and it's complete.