A 22lb recoil spring should work well for the 400 corbon but as Savage sez, the cost of ammo is pricey, very pricey and I don't think it is worth it. Just for the hay of it please take a look at the lineup of Starline brass - I think they have another necked down 40 caliber casing available (necked down from 45 acp) and barrels should be available from one of the barrel makers like Bar-Sto.
The corbon round is an off-shoot of the old 38-45 that was developed for mid-range wadcutter loads for the Army target 45s used in centerfire matches. One past gunwriter named Dean Grennel tried hotloading the 38-45 for better velocity and performance but it didn't go well as that necked down design with such a long neck and short fat case capacity kept blowing cases. Bob Loveless, a knife maker from Ca. advocated for a shorter necked design but grennel felt it was too high pressure and recommended his own concept to Peter Pi of corbon. The corbon load was a direct offshoot of the 38-45 endeavour but uses lighterweight bullets to get the higher velocites. Some of the other necked down 45 designs do much better and do it with heavier bullets. Maybe Starline can provide pictures or schematics of these other case designs.
If you would like to shoot heavier slugs - up to 200 gn - from a 40 bore, you can look to some of the other necked down cases or go with the 10mm - it's a dang good cartridge. JMHO. Mikey.
