i'll hit this since no one else has
first off congrats on getting ahold of an engine block that isn't garbage! good deal
I don't believe any of your efi stuff with work now with the carb (you were asking about sending units and fuel pumps) i've never tackled this conversion myself so im not much help, I like to leave dogs laying the way I find them.
the stock 5 spd should hold up, depending on how much power the new motor is, what veriaty the T5 is, and of coarse how often and how mean your going to be to it. yes you can run full length headers, and I suggest you do if you plan on making anykind of HP.
im not going to make a HP estimate for you, I don't know anything about this motor. Good Luck!