The block is a pre 1985 350, the car is a 1991 RS which I think originally had a 305. In the process of tracking down oil leaks that have formed, I came across the oil pressure sending unit. It is the one above the oil filter with a 3 prong plug. If it matters, the big hex on it fits my 1 3/16" wrench pretty well. It might be metric, but I don't have any other wrenches near that size to check. Given that there was oil around, but not above the sending unit I took it out and discovered that most of the thread sealant was gone. I cleaned off what little remained. I've read here and elsewhere that teflon is not advised for this location, so I put red RTV on it, and put it back in. While reinstalling it I noticed why it never felt like other NPT fittings when I tighten them, the big hex that I drive it in with is hitting the part of the block that holds the oil filter, preventing it from going in all the way. It seems like it should be sealed with the red RTV, but it bothers me that the big hex is pressed against the block. 1) Is this a problem, assuming that it no longer leaks oil. 2) If so, what is the fix? I considered an npt extension, but it seems that I really shouldn't need one. The sensor its self works fine from an electrical sense so I don't want to replace it if I don't need to.

You can:
1) Get an extension for the sender.
2) Replace the sender.
3) Relocate the sender to the oil port near the disributor.

I ran in to this issue a while back with a prior engine swap. My solution was a new sending unit/pressure switch since it was giving a faulty read as well. If yours works, then no problem. I didn't like the way it was contacting the engine block, along with the fact that it only seemed that 2 or 3 threads of the unit were seated inside the port, so I replaced it.

I as well had this same problem with an aftermarket OPSU from NAPA. Didn't want to deal with rigging it to fit properly so I bought OEM from local Chevrolet dealership. Unfortunatley I had to pay top dollar for dealership part but the new unit fit perfectly.

Thanks for the quick replies. I'm assuming that a standard brass pressure fitting is fine? It seems something like this (http://www.palmer-pursuit.com/cart/i...products_id=13)
may be in my future, although I'll just get it from the local hardware store. Is the red RTV an acceptable thread sealant for this? I never liked the other pipe sealant goop for pressure fittings, and I already have the RTV.

If you're looking to extend it out a bit, shouldn't be a problem. I used teflon paste on mine to seal it.