that's a good little truck you've got there <img src="/forums/images/graemlins/kewl.gif" alt="" />

the carbs that were used on the 20R motor that were in the '79-80 trucks don't have the greatest service record. i'm more than willing to bet that your problems are due to your carburetor and emissions system. a good replacement is a weber 32/36, and block off the EGR as it more than likely doesn't work any more. there's kits for both of these things out there.

as for the extended cab, you can swap bodies on toyotas but you'll be in for some work(cab mounts etc). there were no first generation('79-83) trucks available in the US with anything besides a single cab. you'd have to step up to an '84 or later body to get an extended cab. it's all in the amount of work you're willing to do <img src="/forums/images/graemlins/notooth.gif" alt="" />