What motor should i put in my 1973 Pontiac custom sprint?

[Eric]

I need help i posted on here before and i said i had a 400 block and a olds 455 block and some one said get rid of both so what should i build for this car it has a 4 speed m22 in right now and i want that to stay i finaly have money to put in this car so any help would be great