CBF;
Considering the situation, it would be a smart thing to walk away from this job. This sounds like "reverse engineering", which requires you to have ALL information pertaining to material and heat treatment condition. Attempting to fabricate (weld) a smaller guide would be pure guess work without proper design information, and could result in serious consequences.
17-4 PH is readily weldable with the GTAW (TIG) or SMAW (stick) processes using minimal preheat and maintaining the interpass temperature at or below 250 deg F during welding. You have several choices for weld filler metal - an AWS ER630 (17Cr-4Ni composition), Inconel 82 (nickel base) or an ER308L. This is the reason why you need to know the heat treatment condition specified for this guide.