you didnt state which engine you currently have, but being a 94 f250 i would say 351? if you do have the 351, in order to do engine upgrades it will need to be maf due to the sd system not fluctuating with engine needs. the best thing to do is find a 94 or 95 302 and take its comp, harness and maf sensor. when looking for a computer make sure you find one that is for the e40d, some trucks had the 4r70 trans and its not the same.
this link will tell you what comp you need- Ford Fuel Injection » EEC Program Codes
the speed density is not "stuck" to that engine, its just the computer style. so once converted to maf you can add your mods and maf usually adds power also.