76- 4oo engine Can i get 500-550HP stock exhaust

There is nothing wrong with using what you have. Use what you have, buy what you can afford.

Yes, you can use stock rods. Pump gas is possible, 800+ HP is possible with pump gas. Porting WILL be needed with the iron heads. 550hp iron heads are gonna be $1,200-1,700. Aluminum would be cheaper.

But Mike is right, that is just the beginning.