72 Swinger engine swap: power per dollar?

Hi all,
Its been a long time since I was here, finances have changed so its good to be back playing with cars again. I've read all over the internet, and engine swap opinions are all over the place. Its practically a dead horse, I know...but I have a situation and would love input from those who have done it before.
I have a 1967 New Yorker, untouched and in sweet shape. It has a 4-bbl 440 and a 727. I also have a 1972 Dart Swinger. Hot little slant six with a 904. I've recently put an 8 3/4 with 3.91/suregrip in, .920 bars and poly bushings up front. My buddy wants to buy the New Yorker for $1800...I want a bigger engine, to the tune of 450-500 hp. My buddy is an older guy with a lot of connections and says he can get me a big block and tranny. Or a small block/magnum engine. What do I do? Is it cheaper for me to keep the car and relatively -know- what I have? Or sell it and use the 1800 on a different powertrain? Or use/salvage the 318 poly I have? (Forged crank, '62 model so tranny are easy.) Goals are weekend driving/ cruise-in car. I would love to improve the suspension but I guess a BBC would negate that considerably? Opinions/thoughts are appreciated! Thanks.:prayer: