Speedo will be totally independent on your '81. I see people selling 241 parts, you could probably eventually find the speedo drive output parts, to hook it back up later on.
I *believe* the wire you describe is the sending unit wire. There are actually only two wires on those setups, a ground wire that runs to the frame somewhere, usually black, and typically a pink wire for the gauge sending unit wire.
I'd probably not replace the sending unit, unless you KNOW it's bad. (not a fact just because the gauge didn't read right) They aren't really cheap, and not a whole lot to go wrong with them, unless it's really rusted and you are afraid fittings might fall off or leak. You can clean the sender to make sure it reads as well as possible, I believe others have discussed the procedure, as well as myself. You can try searching, if you can't find it, post up again, I'll try and find it.
Fuel pump is external of the tank, bolted to the passenger side front of the block, down low near the frame rail. I *personally* would not replace unless it dies on you. Quite reliable, and easy to get, so either buy a spare and keep it in the truck, or just wait until it dies if you are using the truck solely as a daily driver. Aftermarket parts are iffy in quality, putting a new one on doesn't mean it's going to be any better than the one you take off.
Fuel filter(s) if stock, are in the carb (where the metal fuel line threads in to it) and at the bottom of the sender, that little "sock". If the sock is in good shape, leave it alone, (new sender probably comes with one if you do go that route) carb one is fine to replace, but tends to be a real pain.
Tips for the fuel filter in the carb that work for me is to disconnect the metal fuel line at the fuel pump, which gives you just a HAIR more movement at the carb. With that loose, remove the fuel line from the carb, watching to make sure the nut is spinning on the fuel line, and you are not twisting the metal line up. The nut SHOULD spin while the line stays in place. If not, that fuel line is a pain to find new from all accounts, avoid kinking it at all cost.
When working with the fuel system metal lines, ALWAYS use tubing wrenches, NEVER use open ends. They are made specifically for these jobs, and are well worth buying if you don't have them and need to do this. When dealing with the carb portion for the filter, you need a large open end on the carb fuel filter housing, (it has 6 wrench flats, metal fuel line threads into it, it actually threads into the carb body) I believe it's 1", but in any case, you just need to hold the filter housing steady while you try and remove the fuel line. If the fuel line is stuck (which it typically is) you can let the fuel filter housing turn if you need a bit more give, but don't thread it in any further, only let it thread out, if that makes sense. Once you get the fuel line off, then you remove the filter housing, pull the old filter out, and replace. Take original to store with you to ensure new filter is identical to the old. Be VERy careful threading the filter housing back into the carb. For some reason people screw this up quite often (enough that there is a repair kit for the problem) even though it's not hard. It should thread in by hand easily, and just needs to be snugged up to seal. Believe it even uses a seal around the outside edge, replace it if it does. The carb body is soft, this is why you need to be careful trying to put the filter housing back on.
This is all assuming a stock, unmolested setup as it came from the factory.
