Starter mount oversized to bell holes

[Gonezo]

Existing starter spun its teeth off, purchased a Napa reman, they are identical starters however the old had a recommended bolt sticker of 3/8"-16, 1.45", with a 5/16 shoulder. What i removed from my set up was 5/16, no shoulder, and the bell housing definitely is 5/16. Clearly this lets the starter walk around and shift under torque and that this is just going to repeat if i dont fit the starter correctly. Should i buy some specialty bolt or tap my bell housing to receive the proper 3/8", or is the Napa starter not the correct one for a 390 with an automatic? truck is a 69' CS appreciate any help from you gurus

[carnuck]

Did you keep the original starter?