Since you say a Mustang "unit", I'm going to assume its a ram style, not a rack conversion. If this is the case, I'm not sure how much you can lower the pressure before the valving quits working. Most complaints are from a Saginaw pump to an early Mustang R&P. I know, I've got one. Heights makes a bypass style valve, and some pumps can have the bypass spring changed, to reduce pressure. Whoever did the conversion should be able to address this, either fix it, or can't fix it. I'm also going to send a pm.