kb167 pistons good choice for 318?

Losin the roller would be a fool thing to do. Those lifters are the more expensive of the two and you already have those. They can be reused over and over and rarely go bad. The roller will be head and shoulders above what any flat tappet can do. That engine was designed around a roller cam. Changing that will likely result in failure. ...and no, 400 HP is a very realistic goal.