You cant just swap the heads. The blocks them selves have a different casting. Besides even if just swapping te heads is possible you HAVE to change teh pisotns because they are not clearanced for 4 valve heads. I have a halfway torn down SOHC motor in my garage right now and the clearances are totally different. SO by the time you tore the motor apart and swapped pistons, rods(because DOHC's have forged rods), crank(DOHC's also have a forged crank), and reassembled it, swapped in the DOHC upper and lower plenum it just wouldnt be worth that much hassle, hence why you need a complete DOHC motor. There are several people who have succesfully completed the swap. There are a few who have swapped to DOHC-TT from a SOHC. A 3si member close to me is starting that exact conversion this weekend and they will be taping. If you decide on the swap, be prepared. Make sure you have all teh parts necesary and the know how of what you are undertaking.