Jimmybeam

Greetings all. I've got a problem on a project of mine that's been driving me nuts for far too long. Long story short, I've got an 89 S10 Blazer (I know, wrong forum, but from reading this site over the years I've found there to be much more knowledge here than just about anywhere else on the net) that originally came with a 4.3 TBI. Some years ago I decided it would be a nice idea to swap the stock engine out for a 350 with TBI. I started with a block from a 78 Fullsize my father had given me. I had it completely rebuilt, added 906 vortec heads, and a GM Performance 12496821 TBI manifold for vortec heads. The TBI unit itself is a unit from xtremefi.com, the 50mm one. All of the sensors are new, the wiring harnes is the stock one from the 4.3, and I'm running a 1227747 computer with a custom chip burned from tbichips.com. This truck has been back together for a few years now, but I never got it running right. I did what I could when I finished up the swap, but then I got married, bought a house, had 2 kids, and just didn't have the time for it anymore so it's been just sitting ever since. Over the last week I've had some spare time and decided it would be nice to have a 2nd 4x4 running come winter time so I've been playing around with it again. I dumped the gas and filled it fresh. I also ended up having to replace the lines because they rotted from sitting. Once all that was done, it fired right up. Now that I've given the background, here's the issue.

Ever since I got this thing started (back when I finished the swap) it has never run right. It starts up fine, and idles nice, but once I tap the gas, it starts running rough and will backfire out of the TBI (seems to be from the drivers side) over the entire range of throttle. I can drive it, but it has no power, and just stumbles and backfires all along the way. The timing is set at 0 (set with the engine warm and the tan wire disconnected) but I've played with it a little advanced and retarded and have gotten the same result. The cap, rotor, plugs, and wires are all new. The fuel pump is new, the pressure is 14psi (I've adjusted it higher and lower and still the same result), and I also just replaced the fuel filter this past week. The injectors are the stock 350 injectors, at one time I was running the 454 injectors but it ran awful so those have since been sold. The motor was rebuilt, but since I was told a while back that the valves could be too tight, I backed them off and reset them myself and again have the same result. There are no vacuum leaks. I get no codes either. Compression is good. Today I hooked up my laptop with winaldl hoping to get some data stored, but a half mile down the road the upper rad hose sprung a leak so I had to turn it around and park it before I got anything usable. I probably won't have a chance to get a new hose until this weekend, so until I do I'm wondering if anybody has any ideas they can throw at me... I'm stumped! I hope I listed everything, but I'm sure there are probably 10 other checks that I've done and am not thinking of at the moment. Any help that leads me to a solution would be greatly appreciated!
Thanks,
Sean