What's the best way to fix aluminum intake threaded hole for temp gauge sending unit?

https://www.grainger.com/product/60...p.ds&msclkid=63dd3e1009cc140df1f15887daa036ad You could use a simple coupler like this one with the correct thread for your slug. As AJ mentioned you should trim it to the correct length so the sensor is at the correct depth. Bore the manifold to the correct size and drop in your slug and weld. Pittsburgracer makes a great point on how much real estate you have for these ideas.