15-20hp is easy add a header and small cam in the .425 lift area. Crane used to sell a .420 lift roller cam that worked with all of the stock valvetrain and added all 20 of the HP you want. It was torque oriented and great for the street. You can gain lift with a roller rocker swap. The later ranger rockers have a higher ratio. Moving the power band up over the low 6's takes a solid lifter conversion or not being afraid of hydraulic lifters pumping up and failing. Going to around a .500 lift cam makes it a rowdy idle for street driving and daily use. At one time my daily was a 2300 in a pinto, light flywheel , big valve ported head and intake with a worked over 500 2 barrel Holley. A Hedman header and a 2 chamber
flowmaster. The cam was a Reed custom grind solid . 480 lift that pulled to 8500 rpm. The bottom end was a flat top piston zero deck but nothing special. It was a fun daily but had no low rpm grunt and a 12-1300 rpm idle. The ranger header is an easy swap on a Mustang and only requires and EGR tube change. I have one on my foxbody because I used to live in an emission testing area.