The groove at base of small diameter is for a 1/8"x 1" dia "O-ring" to seal the cap in throttle body. I used Bosch injector out of a 2.0L Mazda junkyard engine. So your bore must accommodate that size injector.
Here you see the adapters with injectors. Note the retainers made from original domes.
This shows #2 throttle body with shaft drilled and tapped, tang in place and ring made from 1" emt conduit. Picture below shows some of the notching needed to fit on TB.
The picture above shows nuts have been added to the ring and the whole assembly has been JB Welded to the TB. I used a 90's era Honda TPS. You will want to make sure your TPS will fit the ring assembly. Pay attention to the hold down screw locations.