Here is a website that has "kits" for various semi-autos. Most everything but the frame. Might be an option. Link is to HK kits.
https://everygunpart.com/handgun-kit...nufacturer=994
Definitely no reason for cheap 9mm fmj to not be the majority of the ammo shot through a .40...in the same fashion as people use a 22 trainer.
Very common misconception with pressure - typical factory loads in 40 run about 3,500-4,000 PSI lower than standard 9mm factory fodder and 5,000-6,000 lower than 9mm +p. 40 is very efficient. The higher momentum generated dictates need for more slide mass and spring resistance. Nothing about it is high pressure or "extra snappy" with appropriate pistols...a G35 with 165gr and 180gr factory loads feels like a G19 shooting 124gr+p.
This year pistol hunting will be using a G35 running an 180gr XTP at 1,180fps with 9mm pressure level.
I recently customized my old Gen 3 Model 22 Duty Pistol. These photos were the first outing getting the RMR dialed in and testing the difference in POA/POI of Federal HST 180gr. .40 and Federal HST 147gr. 9mm. When I convert it over to 9mm I change the barrel, magazine, extractor, ejector, striker, spring loaded bearing. The pistol has proved reliable in either caliber. I sighted it in as a .40 and then with no sight adjustments converted it over to 9mm. I also used 115gr. 9mm as well, which printed about an inch low. I have fired it converted over to 9mm prior to the addition of the RMR and proved it reliable. I still need to do a final sight verification at 25 yds. I do carry it in .40, but would have no problem carrying it in 9mm. I will do the majority of my shooting in 9mm as it is easier on the pistol and me as well. Just my 2 cents for any of you thinking of do a caliber conversion.
Be Aware-Stay Safe. Gunfighting Is A Thinking Man's Game. So We Might Want To Bring Thinking Back Into It.