If your leak down test (ECS is willing to check before sale) comes up good, ECS will be honest with you, just run 7-8lbs for 550-600rwhp on the safe side. No need for the meth at that level. Otherwise, swap out a forged shortblock and turn the boost up.
Yea, youre looking at an extra $5-7K with a forged 402 installed (I would do). The guy has taken really good care of the car though, but more opinions on here will help you decide. I would do it, I would just drop the boost a bit and save for a shortblock. That would be a fun car with 800+ to the tires.