[Lugnutz65]

Originally Posted by jayoldschool

There's the first problem. The stock wire is a resistance wire, and you are not giving the HEI the full voltage it needs. Run a new wire, ASAP, then check that timing.

I have read all the above and agree with the following:
Here's what I think you should do in the following order.

1. You need to run a dedicated full 12 volts to the HEI dizzy. Use a 10G wire or bigger.

2. Now, set the timing to 10 degrees with all vac lines plugged. You can go up to 14 degrees later, but see what kind of power you make at 10 degrees first. You can fine tune the timing using a vacuum gauge.

3. Your 235/70/15 tire is a 28" diameter tire and should be fine if the above issues are addressed first. A smaller tire will treat the symptoms but it's not the cause of your power issues. Also a smaller tire might look too small on a truck. It's a matter of personal taste, but I wouldn't go smaller for that reason.

4. Swap the rear to a 3.73. remember that changing the rear affects EVERY gear. A lower rear end (i.e. 3.42 or 3.73) will increase the power you have in EVERY gear. It will also reduce your MPG.

5. Add an OD transmission if you want to keep the 3.73 and want better MPG. An OD tranny will not affect performance in the first several gears but it WILL reduce your RPMs in OD and thus give better MPG with highway driving. A nice AT for a 3.73 rear diff is the 700R4.