What material to use for a plug on the accessory case of a Lycoming 320

[2013grandsport]

So my IA and I eliminated my vacuum system on the Apache because I now have dual Garmin G5's... So we figured we would lose some weight.
In doing so I have a open hole now in the bottom of the accessory case where some of the routing from the vacuum pump went. It is pipe thread, and I found locally some nice Brass plugs... However as you know this is a dissimilar metal. Is this a no go item? The IA says I should find steel plugs... Isnt that however dissimilar as well since the case is Aluminum?

 

[Tweety]

 

[2013grandsport]

 

[flyguydon]

 

[Tom]

 

[GM.]

 

[Orest @ CYFD]