Rebuilding my 340 and bought the Fel-Pro kit, went to install the rear main seal for the crank,and it fits really really loose. Ordered one from Nappa,AutoZone,advance auto parts and still the same. Are you guys just using silicone to hold it in place and fill the gap or is there something better out there ? Thanks !

The 2 thin halves with the squared off ends that go above the crank/above the #5 main cap OR the rear pan seal with the tapered edges? the 2nd one I am thinking. I have no 340 exp but the #5 rear main cap is different on a 318 than it is on a 360 & may take a specific one. I think a person could after mocking up if one is too long & wont set flush in the groove & that is to cut a piece off away from the tapered edges of course to shorten it so it will set flush in the wide shallow groove on the main cap then seal that junction with some silicone. otherwise here is a BTT for ya. EDIT How loose is it? my 360 one was a slight bit loose, like it did not want to lay dead flush in the wide shallow groove but with some sealant under it it sealed up OK (no leaks).

Having issues with the lower seal .... it literally falls in the groove and wiggles back-and-forth. At this point I guess I'll have to pack the groove full of silicone to take up to gap.

the seal half or the rear oil pan seal? I wrote myself a note to check fitment tomorrow with what I have & I will holler back.

I checked the 2 thin black halves that go around the knurled journal "the rear main seal" & the halves fit flush in the block/#5 main cap on my 360 but one half was too big for a 318 #5 main cap that I tried it in (I have no 340 stuff) so the other half would for sure be the same & be too big to fit in the block recess. "The rear pan seal" that sets in the wide shallow groove in the #5 main cap to seal the main cap to the pan, set in there fairly flush on my 360 #5 main cap but the 318 cap I have has a flat wide surface as opposed to the wide shallow groove that my 360 caps have so I could make a comparison. here are some pics. I'm thinking the 340 might be the same here as the 318. EDIT these above are a 360 cap & I will see if I can get some better pics out of this complicated Samsung camera (at least it ain't caught on fire).

What year 340?
Does yours have the extended tangs on the bottom half of the seal? Probably not, as you said they look like each other

It's no wonder there are so many issues with sealing rear main's

"It's too big"..... cut it!
"It's too small"..... silicone it!
"I don't want it to leak"..... silicone it!
"I'm not sure what to do"..... silicone it!

Dude, you have the wrong seal for the application. Don't care where you got it from or what brand it is it's either the wrong part number or mispackaged.

Do yourself a favor
http://www.manciniracing.com/su340remase.html
(description says silicone for whatever reason, it is in fact Viton)
Proper fit, no sillycone except where the main cap seals (if used) contact the rear seal.

Agreed, you shouldn't be cutting,fitting,sealing etc, wrong parts,