And the lord said:
"Weld the damn thing on!!"
If you want to be flexible for later trials, weld a flange to the plenum.
You bolt an adapterplate to the flange and bolt the throttlebody to the adapter with sunken boltheads.
O-ring groove in the adapter will seal fine.
When settled on a final config. scrap it all and weld the d**n thing on....
Thomas :)