Depending on the thickness of the casting, that should work OK. If it is too large to fit in a drill press or milling machine, important thing is to get the tap vertical and use a brass plug. Search around and get the plug first, they make them with recessed six point or raised six point. Raised 4 point would be a poor choice.

Make a small fixture to hold the tap vertical to the surface. Get tap OD from the smooth(upper) surface, make a small block with hole size of the tap and bolt it down with the adjacent threaded hole. Aluminum or even some hard wood would be fine. Use some tapping fluid and check it frequently with your plug. Use some setting thread material after cleaning with acetone to remove all vestiges of tapping oil. Put some grease in the tap grooves to catch filings.

Ed

Last edited by EdPruss; 03/20/2021 7:20 PM.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.