Just a suggestion since these offset brackets are not readily available.
What about using a couple of 2-1/2" 1/4" bolt sleeves along with 3" or 3-1/2" 1/4"x20 bolts/washers/nuts to mount an original "flat" style horn bracket (readily available from regular vendors) to the original intake mount holes?
The 1/2" OD spacer sleeves should still clear the intake and relocate the horn forward of original position to clear the oil filter housing.
See attached "McMaster" source screen shot for sleeve info. Found these as least expensive, but also available in black anodized aluminum or steel if you want a different look (more expensive). You can also find similar products from other sources, possibly even a local hardware store.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)