Bjorn's comments were not about cosmetics. 400s need more airflow to adequately cool in the Firebirds, or significantly larger/more efficient cooling systems. Do a search on 400 cooling on this site and you will find many references to this.
Block holes will match up so that all 1969 accessories and pullies will mount. The only possible exception is the alternator bracket, check your left head for the appropriate hole for the pivot. You may want to replace the fuel pump with a '69 400 fuel pump (with or without return line as appropriate) and check the diameter of the stock fuel lines from the tank to be sure they match your pump.
Use the Firebird oil pan.
The blocks and heads are externally the same dimensions, so that if you have no clearance issues now you will not have clearance issues after the swap.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
