Type of rear end on 64 impala

is there a way of telling what type of rear end is in this impala ss that i bought. it seem to be geared very low. the car has a heavy cam in it and a stall in the transmission. it has a 400 engine bored 30 over. i was told if i put another rear end in the car i will be able to drive the car on the street with no problem without blowing the engine. any other suggestions on how to make this car better for driving on the road without blowing the engine. what type of tachometer should i buy. thanks everyone