So I am looking at 75 duster with a 340. I want to take out the 340 (no i'm not crazy, I already have a 340 and don't need another) and swap it with a slant. the car has a G in the VIN, so I think it's an original V8 car. The 340 isn't original. So the question I have is can I put the slant 6 on the V8 K frame or do I need a new K? Thanks guys.

Going "by-the-book"... you need to swap K-frames.

From my personal experience, special engine mount brackets can be made to mount a SL6 to a V-8 K-Frame.

See the fabricated, spool type engine mount below, as an example.
The same process can be done on a "pillow type" engine mount bracket.
DD

Ok so I'm a little confused. I see those spool style mounts available, and my 73 dart had the same type of mount for the v8. So asside from the v8 mount being wider, what's the difference. I'm just confused.

The above looks to be a stock 73'-'76 /6 mount that has been welded to a cut-off upper end of a similar mount or other bracket on the block to extend it out and down to the V8 K mount position. (Hence the term 'fabricated', I presume.)

The statement above is correct. we simply combined the SL6 mounts with the V-8 units... using some cutting & welding.

What I do is mark the "old" engine's crankshaft center-line position onto the K-member, before removing that engine. (clean a spot and paint / punch a mark on it)

Pull the engine and place the new engine into that position.
Level it out & use some wood blocks to hold it in position.
Start measuring, cutting & "fitting" brackets to fill-in the space between engine to K-frame.
(Basically, SL6 top and V-8 bottom bracket pieces)

Once the pieces touch in a couple of spots, tack weld together, while in position, then remove and finish weld on the bench.
( Do a few "trial fittings" and remove the supporting wood blocks, as you go, just to ensure clearances)
DD

I thought the slant six mounts were spaced differently fore and aft in the car as well? we were looking at putting a slant on a 99 dakota frame that had a 318 and as far as I can tell i will have to cut and reweld one of the two side mounts as well as marrying them together like your showing.
If i'm missing something please let me know because id much rather do all the fab work in the mounts.