ring and pinion

i just got a 1970 datsun 510 it has a 1600 with an 1800cc head on it. its a great little car but its soo slow. it originally had an automatic in it and had a 4 speed when i got it. and im putting a 5 speed trans in it tomorrow. but i dont know if this is going to solve the speed problem. so does anybody know where i can get a set of taller gears for my rearend? ive looked at about 20 different places and found nothing.. so i would be grateful for any help and tips on making it go a little faster.