Max rpm 360 magnum ???

Brooks those stock heads are going to run out of breathing room well before that. If I remember correctly A totally stock 5.9 could be safe up to 5500 even though the ECM was good to 6500. At 6300 you’ll be way past the power band and on the decline anyway and in my opinion in an unsafe range with potential valve float. Not sure what you have there for other goodies or how you use your car. If it’s completely stock with the mild Hughes higher lift cam and no head work or head upgrade I wouldn’t bring it above 6000. My 5.9 retains the stock heads with probably the same Hughes cam you’ve got and an edelbrock air gap with a demon 625 carb. I don’t bring it past 5800 but think it could be safe to 6000. Smarter guys than me will chime in.