I have a 1998 4.3 liter K1500 4L60e tranny that might need a new boost valve. The tranny lost most of reverse (moves a little ) but all forward gears shift well. I see there are .500" & .490" boost valves but I don't know which valve will fit. Any help would be greatly appreciated.

Thanks

 

The Sonnax 4l60e-LB1 (note the "1") will be your best bet. Most trans shops will have one in stock or can order it from their distributor for next day delivery. Also available online and ebay, although shipping costs may pump the price. I see it on Amazon for $27 with free 3-4 day shipping.
It doesn't matter if the valve is .490 or .500 since these "kits" come with the sleeve into which the valve fits.