Blazer Forum - Chevy Blazer Forums - TH350 won't downshift from 3rd

I have a '76 K5 with a 350ci and a TH350 in it. I've tried adjusting the kick down cable every which way I could imagine and always run into the same problem. It will kick down from second to first just fine but never from third. It just sits there in third when I come to a stop. I have to pull my shift lever down to second or first before it jumps back down. Is this something internal in the tranny or am I just doing something wrong? I was thinking about just ditching the auto and swapping to a SM465 if I can find one for a decent price here in Albuquerque. But I don't have much experience with this. Is it pretty difficult or can I do it myself without too much pain? Thanks for any and all suggestions in advance!