Does your state even check emissions on older vehicles? You might check. many states have a provision for vehicles over a certain age being exempt. If that is the case less is best when it comes to emissions equipment. It's often easier to take a lot of it off and remove 40 year old rubber lines to eliminate vaccum leaks and such. Just food for thought.
this exactly. If you don't have to pass emissions, then less is more. Yes, like SirRobyn says the stuff that comes out of tailpipes is technically bad for your lungs and brain at huge concentrations, but if you're not in the habit of running the truck in the garage for long periods of time or making out with your tailpipe, you and everyone around you will be A-OK. You probably won't notice
much of an INCREASE in power or fuel economy (though technically there will be a slight gain to both), but what can HURT both of those is vacuum leaks, drag from belts and worn bearings, etc. If the stuff is already off and you have no legal requirement to get it working again, then buying the missing parts only costs more money, and installing them only causes more chances for headaches. When brand new and it was working fine there would be no good reason to remove it, but now that these trucks are ~40 years old the more crap going on under the hood is just more things that can potentially fail and make your truck not run right or even get you stranded worst case scenario. Personally under MY hood, the only things I need are an engine, fuel system, simple cooling system, and a simple charging system. All that other crap can p!ss off as far as I'm concerned. Easier and cheaper to own and maintain, less chance for breakdown, and literally zero downside unless you live in an area that forces the issue and you're the type that enjoys following silly rules...