I just had to get a new radiator in a hurry.

got it from radiators.com

I have a 75 280z 4spd

The radiator has 2 small nipples at the bottom towards the center on the side facing the engine. I don't recall my old radiator had these, (it is still at the shop that said they wouldn't bother fixing it,)

Are these for lines for an auto tranny and need to be plugged or do I just need to fumble around a while longer to figure out where they go?

Sounds like automatic transmission lines, from your description. If so, you won't have to do anything with them if you have a manual trans. These lines typically serve two purposes....1- helps bring an automatic trans up to operating temp....2- keeps the tranny at manufactures recommended operating temp. At least thats the way I always understood it. (basically a trans cooler in the bottom of a radiator). Hope this helps you out!

I am assuming then that these do not need to be blocked?

nope.

That is a correct assumption.....they do not need to be blocked!!!