A 100°F difference in temp in 1 cylinder would be a big deal for me.
Start checking for vacuumleak in that cylinder.
(You don't have the PCV hooked up in that area of the intake do you?)
My propane fed 360ci has cyl.no 6 idling with 40°F warmer because of a less then perfect valve seat seal.
The other cylinders only vary about 10°F between them.
If you have an automatic you load the engine against the convertor at you chosen rpm and check all the cylinders again and see if the big difference in temp follows along or evens out.
