The main problem with those late model big blocks is that they had very low compression. The pistons were way down in the hole. And that made them very prone to detonation even on the gas you could get then.

And I agree that the pistons and heads look like they have not been burning clean.

Use the money that you would have spent on the FI on the inside of the engine now and get a good carb like the Street Demon. You can always add the FI later if you want. It will be better and cheaper by then.