Sounds good! I will echo mswyka. A post a few weeks ago prompted me to look at my '79 parts manual, which showed, to my surprise, that both the 340 and the 440 Invader shared exactly the same part# for the y-pipe and the tuned muffler.
This had to be a slight, economic compromise on Kawasakis part...hard to imagine that this arrangement was perfect for both motors, although they did share the same 7800-8000 RPM power peak....but it obviously worked well.
Without looking, I know the '81 440 used a new part # for the tuned muffler. I think that the '80 Invaders did also. Kawasaki changed to this slightly improved design...it was probably only good for a horsepower or two, but enough that Kawasaki felt it worthwhile to make the change and add to the "headaches" of part inventories, etc.
That '80-'81 pipe is probably the "hot set-up" for any stock Invader.
You have probably already thought of a couple things to consider in making this switch from a 340 to a 440....The drive clutch was identical, except for the fact that the ramps for the 440 carried a few grams more weight, in the form of tiny washers, to account for the greater torque from the 440.
And, the gears were significantly different for the 340 versus the 440.
I have been a bit amused by a chart in the service manuals for these sleds...This chart shows a cool breakdown for various gears. The 440 Invader used 21/39 gears, which was supposedly good for a "theoretical" top speed, at 8000 RPM, with the clutches at a .96 overdrive, of 88 mph.
The 340 came with 18/37 gears...good for a "theoretical" top speed of 79 mph.
I don't know what anyone else thinks, but a stock 440 Invader would have to be incredibly well dialed in to achieve a true 88 mph on radar, or GPS...never mind what the speedometer says...perhaps slightly downhill, and with a good tail wind.
Pay no attention to the folks on Craigslist with a '74 something or other that can go 110 mph! LOL.
But, the gearing comparisons between the 340 and the 440 remain valid..
I have read on here several comments from owners of 340 Invaders...how much they admire how quick these sleds are...certainly the 340 doesn't have the horsepower or torque of the 440, but the lower gearing of the 340 certainly contributes to its "quickness".
Good luck with the conversion to a 440....if you retain the stock 340 gears, that will be one quick Invader from 0 to 60!