gday i am about to buy two Ford XA sedans, real cheap

i was planning on using 1 for spare parts and the other to restore.
but it has a "3 on the tree" transmission and i would like to change to a 4 spd manual if i could how hard or easy would it be?
and whats the fuel economy like with the 250ci straight 6 engine? on petrol and gas. cause im on me P's
and tell me if the ford xa would look dodgey if i put a spoiler on it and a scoop