Best BB bang for your buck?

I'm wanting to turn my /6 73 duster into a BB. I'm on a tight budget, I can't put much more than $1,500 into the BB, that being over a period of time. My question is, what motor can you modify for (relatively) cheap to get the best bang for your buck? I read a lot on this site about the 400 being a good motor to mod for cheap?

Thanks & Cheers!
Drake