I think a few of the above replies didn't notice the part about the spacer. This calls for a "Customized Custom Valve" so I'd also suggest getting Chuck on the phone. He says he's the manufacturer so he might just be able to make one for you.
If you had a "long" brass stem and needed "longer" it should be possible to braze a stretch piece into the middle of it. Just remove anything that isn't metal during the process. Or silver solder, even soft solder with flux like copper tubing, just allow 1/2" overlap at the joints. Come to think of it, a good epoxy would make for a fix you could do at home.
Call Chris at U-Joint in NC where he builds one-off 4x4 conversions. I'm sure you've tried offroad shops and commercial truck tire shops.
Last resort, a good-quality stainless-braided flexible Air-Less Extender. I think the one-piece custom brass valves are the ultimate, but many of us run extenders and few have problems. Just so long as that inconsiderate installer installed a METAL valve with the new tire.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB