Horsepower = RPM x Torque / 5252
At first glance it would appear that your engine would be making about 535 HP. But since the torque was measured at the rear wheels and I assume that you were measuring engine RPM and not wheel RPM that this is not the case. What you need to due is divide out the rear end ratio, and the transmission ratio and multiply the torque converter slip to get the RPM the rear tires were turning. My guess is top gear with a 1:1 ratio, a rear end ratio of 3.5 and a torque converter slip of about 5% (pure wild guess) your wheel RPM would be: Engine RPM/(3.5)*(1-.05) or RPM/3.68. So your car was putting down to the ground about 145 HP. How this relates to your car can be seen at
http://vettenet.org/torquehp.html