The original owners manual says AC R44 @ 0.035 gap

The 3" thick official Helm service manual doesn't say.

Haynes & Chilton Manuals, along with 3 national part stores recommend
AC R45 @ 0.045 gap.

Who is right? does it matter?

I've done light modifications (Open air filter, headers, no smog). Should I use something else with these changes (that's all the modifications I'll probably ever do, I might go back OEM stock someday for originality)

I've done both over the years approximately every 30,000 miles. Car just turned 230,000, running great. I went with the R44 at 0.035.

 

The factory gap for the 86 LG4 is 0.035.
You'll be fine at 0.045, it doesn't matter that much. I personally wouldn't increase the gap much farther than 0.045.


-Matt

 

44s are one step colder than 45s. It will be ok, as long as your engine isn't using oil. If it is, a warmer plug can help burn off any build up on the electrode.