To quote him, he said the aluminum block would "move around
Tell him you want it to "move around" . . . that's why you're putting wheels on it.

Is your friend an "old wife?" The trouble with depending on "old wives tales" is that they often aren't true.

Do you NEED an aluminum block to get enough horsepower in a big block Chevy for street use? Of course not. You can get more than you can use with an iron block and iron heads. However, an aluminum block and heads will get you lighter weight and a bigger "WOW" factor. If you can afford an aluminum motor and you WANT an aluminum motor, get an aluminum motor. If it's used, though, just make sure it's fully inspected - of course, that's the rule for any motor, not just AL motors.

Final thought. You might want to reconsider that "definitely knows his stuff" line of thought.