I'm having a tough time fitting the oil filter using the 90 degree adapter with the headers. Is there such a thing as a straight adapter for a 340? Where would I get one?

can you angle the 90 deg adapter so the filter clears? and there are shorter filters avail. there's the std flat plate w the center nipple along w a short filter

I installed one on a 360 Duster with headers. I did not tighten the center bolt until I rotated the adapter for the best clearance between the headers and the frame/firewall.I then tightened the center bolt and all is well. If you need to replace the gaskets for the adapter they are available. There is a large one for the base and two smaller ones for the top and bottom of the bolt. I am able to use the full size long filter and it works great.

Never had to use an angled adapter with them. Always took them off as more of an obstacle than they were worth. Just use the plate and double filter mounting nut and be done with it. Use a smaller dial\meter filter if you must but they were developed for use with the short standard diameter filter of the day. There are a lot of filers with the same thread and filter sizing out there that are smaller in diameter and a lot longer to give you more filter if you don't want to use the smaller short one.