So my drill bit from Begi is 14.5mm. My tap from Begi is 3/8 NPT.

Isn't 9/16" drill bit the correct one for a 3/8 NPT tap?

9/16" = 14.2875mm

14.2875mm vs 14.5mm

1) Is .2125mm negligible? Will the 3/8 NPT tap be fine?

2) Does this oil Pan fitting on ANPLUMBING sound right?
982311 -10AN male to 3/8NPT male 45 Deg. Elbow, Pipe Thread To A.N. Adapter $12.95

Thanks.

 

Remember that the T stands for tapered so 14.5 will be fine. You dont have to run the tap right through, just enough to cut the threads. Equally dont tap too shallow and end up having to put the tap through again because its a bitch to find the old threads and you could end up cross threading it and making a hell of a mess. So dont tap too far and dont tap too shallow I'm sure you'll work it out

And yes the fitting sounds perfect.

 

Basically get the tap in a quarter of the way, test fit the fitting. If its not deep enough in the pan go again with the tap, then test fit again.

 

You COULD test the depth you need in a piece of aluminum away from the pan first.