puzzle pieces

I'm building a new engine (318) for my '67 Fury III
I picked up a set of RHS heads (20301 LA-X) and am having a difficulty matching up the valve train parts.
I have a set of 1.6 PRW bushed with roller tip rockers.
Here's what seems to be the problem:
The RHS heads use 318 magnum valves and springs, but the rockers hit the springs and don't clear the spring retainers.

The question is what size (dia.) valve springs do I need and/or do I need to come up with a set of beehive springs. I've seen some advertised, but they don't specify whether they are for an LA valve train or a magnum valve train.

Author: old_phart