impure

i recently bought a 5.0 mustang its the 89 lx with a 90 engine in it. it wont shift into third gear and when done manually it jumps up about 2000 rpms therefore it is not even catching third gear. so is it shifted electronically or mechanically? i think it is done mechanically but i dont know. also will i be able to switch the tranny from the c5 to the c6 cause my uncle will give me one for free. i need responds fast so i can throw the trans in this weekend