Hi Mate
I just went through this too!
Firstly, you need to identify your pump brand. It could be either a TRW or a Federal. See this pic -
If you have a TRW (sometimes also called Thompson) there are several sub types, distinguishable by the end of the shaft being either plain (also called small TRW), or slotted or having a hexagonal recess (and also called large TRW).
I recently bought a kit from Rockauto for my small TRW pump on my 62 Dart. (Mine was Gates kit 350370).
If it's leaking between the cast iron pump body and the sheetmetal housing (and NOT the shaft seal), it's likely just the body o-ring needs replacing which can be bought for a few dollars from your local bearing and seal shop. Eg -
http://www.sealingdevices.com/o-ring-sizes (Mine was a 2-245 on that chart.)
If so -
Remove the PS pump from car. (Careful not to bend/break the return hose fitting in pulling off that old hard hose as it's only soldered into the housing. Ask me how I know?!)
Drain it.
Remove the mounting bracket and bolts at the back of the pump.
Pump body can now be pulled or knocked out of reservoir. Take care not to bend the reservoir in any way!
Dont lose gasket that is sandwiched inside between reservoir and pump that seal the bolt holes.
Remove and replace o-ring in groove on pump body.
Clean all and re-assemble.
If it's leaking from the shaft seal, then you need a kit, and fitting that is a whole other bag of beans. Just removing the pulley can be a real prick without the right tool!!!!!
Good luck, and please update as to how you get on!