I have never seen an origional 69-70 road runner engine that was not an HP, and orange.

I have seen several origional 69 and 70 a/c automatic road runners with orange HP engines,
and even have a late 69' 383 a/c auto block in my garage that is an HP and has origional orange paint.

Because of this, I think you would be safe to paint it orange and use the HP block, but I am not an expert.