TR2/3/3A Iron or aluminum plug in top of head?

The original aluminum head plug in the TR4A engine in my TR3 failed and let water leak into the oil. What a mess! With the help of some Triumph club members we got the remnants of the old aluminum plug out (what was left of it) and cleaned the threads in the head. I have bought a new aluminum plug from Moss, however, since I have always heard that it is not good to use aluminum and iron together (dissimilar metals), I have fabricated an iron plug from a 1 inch diameter bolt with SAE 14 threads per inch, per what Moss Tech Support told me is the correct thread for that top plug.

I have two questions:
1) Is it best to use my fabricated iron plug, or the original type aluminum plug? See attached picture of fabricated one.
2) What sealer should I put on the threads of the plug? ( I was thinking of using Permatex Aviation sealant, but not sure that is the best)
Thanks for the help!
Bob

The iron plug should be fine, I've had heads reworked where they tapped to a pipe thread and put in a cast iron pipe plug. Not sure what sealant is best, I think last time I just used a pipe sealant but it's been a while.

What some people have done is chase the hole with a 3/4"-14 (NPT) pipe tap and put in a 3/4" brass or iron pipe plug. Use whatever pipe sealant you like.