How much work is it to replace the heater core in a 02 blazer?

Just finished rebuilding 2004 Dodge Stratus DOHC 2.4L engine (timing belt broke). Just about every engine part was either replaced, rebuilt, or reworked. It starts (very difficult) but cylinders 2,3 & 4 are misfiring. #1 plug is clean but the rest have a heavy carbon residue & are gas soaked. Double & triple checked the timing, both marks on the cam pulley are aligned at TDC. There appears to be enough spark. Scanner only gives a code P0016 (Cam/Crank Position Correlation Sensor A - Bank 1) & it's only reading temp sensors, not any other vital sensors. Normally if the Cam or Crank sensor is not working the car won't start at all? So either there is one other sensor bad or when the shop pulled the crank sensor gear off to put it on the new crank, it was put on in the wrong position (is that possible). However, if it is anything like the flywheel, it can only go on one way. Any ideas? Help? Thank-you.