Truck Backfiring Don't know what to do

I have a 1990 Chevy C/K 1500 4X4 with a V6 and it just started to backfire constantly.

for a little bit after the first starting of the day its fine

then when i go to drive again it seems like it is loosing power going into
1st gear and will putter for a bit, then when i shift to 2nd gear it looses power and starts to backfire until it get going, 3rd, and 4th gears do the same thing.

I had recently just had the valve seals done and i know it needs a major tune up I have also noticed that the cadilac converter starts to stink going up hill on the freeway

i really aprciate any help

The converter usually stinks because it is overheating. Both that and the backfires are likely due to unburned fuel getting into the exhausd.
So, the question is: Why is unburned fuel in the exhaust?
If it loses power, the fuel is not being burned properly.
Try a tune up, as you state, new plugs, wires, distributor cap and rotor, set the timing AND change the oxygen sensor. It might just be one of these parts, but if yiu say it needs a major tune up, don't cheap out. Do everything properly and save yourself gasoline and frustration.
Also, scan the computer for codes, to see if anything else has come up.

Sounds right to me. A tune up will probably do the trick. Dont go cheap on the plug wires at all. A good set of wires usually runs about $30+ at a parts store. I recomend Accels, last a long time and look cool too!

Something else that may be causing the problem is your cat itself. When I worked in the Chevy garage I had a truck come in that was backfiring through the throttle body and didn't have any power. I tried everything under the sun to figure out what was going on. Then for some reason I decided to drop the exhaust from the manifolds and started it up. It had all kinds of power and never missed. It ended up being a clogged cat. A lot of people think running Premium is the best way to go, but in all reality some vehicles were not made to run it cause it will clog up converters.