There are two 1 5/8 inch freeze plugs on the back of the block,I keep them in stock at the shop because it seems like they are the number ones to go bad because of lack of circulation. Best time to change them is when the tranny is out. 
i find it funny that you say that, ive had about a hundred small blocks, never had an issue with those plugs..had one blow out of the back of a head once (messy). Ive changed a bunch of the side ones that corroded out, but being the rears are straight up and down they dont collect sedement like the ones on the sides do, creating more rust. also they arent exposed to the elements on the outside as the side ones are (road salt rain water ect). ive actually only had one front block plug leak, cant remember its location but its near the fuel pump. had to pull the alt to get at that one, it was in a 2wd pickup an they get alot of crud on the front of the engine when people leave those rubber flaps out.
