What year block is it? 302 heads makes it sound like an 80's if those heads came with it. If it's a factory roller block then you can grab a factory roller from a bone yard and have it reground. Or, since you're probaby looking at a complete rebuild anyways just grab a roller motor.

Even if stuff is in stock you're going to be looking $1500 to retrofit a non-roller block between the cam, lifters, springs and rockers. Maybe more.