How's agoing. Just starting to get into Nissans. Picked up a rb20det for a pretty killer price. Came with everything except the harness which is fine. Any tips for what I could or shouldn't do? Was planning on putting it in a 78' Datsun PU

What do you want to do?

First, I would whip out a measuring tape and make sure it will fit in the first place. I've seen RBs shoved into Miatas so it probably will go in, but I'm not sure you won't have to use electric fans.

Next, transmission? Do you have one that will bolt up or did you get an RB bellhousing that you could swap onto a KA24 box?

Unless you know that the engine has been properly rebuilt, you should figure that into the equation. That engine is over 20 years old and know that it has been beat up. Rebuild it.