Hello, this is my first build and I'm new to a lot of this. I have a 403 that I would like to see somewhere around the 400hp range. This is not a drag build or daily driver, I just want something that is fun to drive and has that extra power if you want it. My plan as of now is: add main girdle, swap to aluminum intake manifold, headers, large aluminum radiator with possibly an added fan, performance water pump, and high flow thermostat (160 or 180). From what I've read the 403 has a tendency to overheat so I want to do whatever I can to help/prevent that. I know that I need to increase compression, but I'd like to do so without having to buy new heads (ideas?). I'd appreciate any suggestions on pistons or anything else that I could do.
Thanks

 

You can get the compression you need with KB Hyper pistons and your factory 4A heads. They have a better ring pack and are a bit lighter. You have better rods but your crank will be questionable at the HP and more with the 450+ ft-lbs of torque it will no doubt produce. Another problem is, those 4A heads flow like crap. The intake side isn't horrible but exhaust is pretty bad, needs a lot grinding along with bigger valves. A set of out of box Procomp heads would give similar compression and flow better, as would any iron BBO head except the J head. Good luck, your goal is easily attainable better heads, more compression and cam.

 

Main girdle (nah, ARP hardware and double check clearances)
Performer RPM intake 7111
Long tube headers, yes
Radiator, yes
Inexpensive alternative to the fans is using a Mustang, Taurus, Windstar, or Camaro fans and a Vovlo double relay setup.
Thermostat, 180*, no lower.
Only way to increase compression without buying new heads is using a flat top piston. Keith Black makes a .020 one.
Stock heads are still not very desirable. Look into 3, 4, 5, 6, 7, 7a heads, cut them at least .024 to make up for the thicker head gasket.
Must drill square the head bolt holes to 17/32 and enbiggen the dowel pins to 1/4" (? can't remember)
Speedmaster makes a cheap stainless valve for them at 2.072/1.680, just need to double check spring height with the new springs.