I’m currently having a hard time getting the timing to adjust right on a new 330 HP crate motor I put in my Chevy one ton and my fuel mileage is in the dumps compared to my old 290 hp factory motor. I understand the computer does the adjusting you should put in on TDC but the thing in still no fuel mileage and the get up and go is still slow does the computer has to be gone into to adjust something more. I have a code reader that gives me timing  and temp and dwell but the numbers just won't go to factory setting. HELP.  

I'm gonna guess your '99 has the Gen I small block?

When you set the timing, be sure you have hte timing bypass plug disconnected, otherwise the timing will be screwed up. As for what to set the timing at, don't give it any base advance or retard, set it straight up at zero. I've got a '92 Silverado, and it's not happy unless the base timing is set at zero.  

Hey mammcdonald, the ignition system is completely different on your 99 than that 92, disregard that advice.
On some of the ignition systems similar to yours you could go to the signal wire on the 3 wire connector coming out of the distributor. It is  a hall effect sensor. anyways, you go to tdc of the compresion stroke cylinder number one and then adjust the distrubutor so the signal is on the rising or fall edge, or, just where it goes to 5 volts or just when it drops to 0 volts.
 On some other ones you have to do it with a tech2.
I can get more specific information on what your up against from service information I have access to. I am at home right now. I could get that 2 you tommorow.  
  It would be interesting to know what trouble codes that thing is setting. I thought that distributor shouldn't even turn, maybe I am thinkin of the v6. let me know