What size is the adapter needed for the 3.4(pushrod). I've seen one article that used a 5/16F to 1/2M from Lowe's, but Lowe's only sells 3/8 to 1/2. I've seen another article say a 1/4pipe to 5/16tubing adapter is used. Anybody know for sure what size the adapter/reducer is? I don't want to spend a lot of time driving back and forth to the hardware store. I don't have the engine yet, so I can't look at it and measure.

Well, if you are talking about 5/16F threads, there is no such thing. All NPT sizes are factors of 1/8's. They do not get any more specific than that.

If you are going to be using a stock Fiero oil sender, I believe that the end of it is 1/8" Male NPT, but I am not sure on that. I never got a real good look at it when I was building up my engine but judging by the general size that seems about right. The fitting on the block near the oil filter is a flare size though.

Yes he is right the oil pressure has 1/8 NPT a company called Equus they list it as a Oil Pressure & EQU6848 1 5.99 Electronic Temperature Metric Adapter Kit most newer gm engines now use a 16mm x1.5 thread for the oil pressure senders. I fabricated a home made one before I found the kit here is a pic. Autobarn.com sells the kit if you are interested it is like six bucks.

That kit looks very easy to make. Just a 16x1.5 x 1/8" NPT bushing with a street 90.

I remember a while ago I had to make a 16x1.5 x 1/8" bushing (exactly like one in pic) for my Volvo with the B230F engine. I needed to put a different oil sending unit in, and the new one was 1/8" NPT. Luckily, I had some hex brass stock, and I turned down a piece to thread and then drilled it out to tap. Worked out great.