" It looks like I`d have to cut it down but it might work. "
____ Right, and for fitting to a stock Monza setup, it certainly won't work without further fooling-around !
And in my opinion, it would be a certain shame to destroy a good-usable/stock 450-Scr intake-hose in an (uncertain) attempt to make it somehow work for an otherwise stock Monza. _ And unless you're willing to accept a cobbled-up (& likely leaky) setup-affair for your Mona, it would be better to go to an auto-shop & try finding some suitably sized radiator-hose to try accomplishing the same function (about as well).
__ It's been over a couple decades (if ever), since I've had both the 450-Scr and Monza intake connector-hoses in my possession at the same time but, I have the impression that the O.D. of the 450-hose is slightly larger (maybe 3 to 5mm, if I'm right about that), than that of the Monza's. _ So before-hand, I suggest that the smaller-O.D. -(as the pleated rubber-hosing obviously has it's min. & max. OD.s), of the 450-hose be checked to see if it will even possibly fit-into the I.D. of the air-outlet opening-hole in the Monza's air-filter/side-cover. _ Cuz otherwise, the filter-cover's outlet-hole may need to be slightly enlarged in order to prevent the rubber-hosing's O.D. from being pinched & wrinkled (causing at least one air-gap leak).
Even with the Monza's original/stock intake connector-hose, it was still somewhat of an issue to fit the opening-end of the rubber-hosing into the air-filter/side-cover's air-outlet hole without any pinched-wrinkles left developed between the hose & air-outlet opening/hole,, which then allowed some more-probable air-leakage. _ Only when that stock rubber-hosing was new & unused, would it then fit nice & properly.
____ In the past, (if I really cared about good/complete air-filtering on a Monza), I'd then use a Dellorto air-filter from another Italian-bike's carb.filter setup. ...
Back in the late '60s, there were many different Italian-bikes with such varying air-filter setups, to borrow from for such deficient Duke-models.
A couple years back, I sold one of the last of such I had to a fellow-member here (Tony),, (it would be nice to see a picture of Tony's Duke-project of which he must've finally fitted it to !).
I've posted (below) a couple pix of the very-same filter-body (with Scr.type cover), which I sold to him,, (as well as a picture showing a Dellorto-carb fitted with an other/smaller type of filter-body).
__ You could try searching eBay for Dellorto-made filter-bodies made for various smaller Italian-motorcycles such as those built by Benelli, Guzzi, Laverda, H.D.Sprint, etc., etc.
Here's a link to such an example (which I believe is just-like the one pictured in my 2nd/center posted-pic, [except without the Scr.type cover])...
http://www.ebay.com/itm/NOS-Indian-Dell ... 05&vxp=mtr ,
and here's another link to another example (which I think may possibly be similar to the filter-body fitted to the D.carb pictured within my 1st/top posted-pic)...
http://www.ebay.com/itm/Dellorto-SHA-Ai ... e9&vxp=mtr , whether or not THIS-one's threaded mounting-hole is large enough to fit your Dellorto-model, should be inquired of (since it MAY be too small).
No-doubt there are others to be found on eBay (later if not today).
The mounting-hole for all THESE filter-bodies is smaller than that made for fitment to the 27 & 29mm SSI-carbs of the Scrambler-models, and therefore are not interchangeable.
____ Hope this helps you make the best decision !
Hopeful-Cheers,
DCT-Bob