[Lynn Eades]

The lead plug is easy. I cant really remember which calibre, but some of the guys use cast lead bullets, which are easy to make(.45 cal?).
Re retention with a split pin, it worked in service, and I concur with Alex, that infact there is virtually nill end load on the pin in normal use.
A thought, that sits in the back of my mind is that anyone wanting repo track, would want it to look original.
The track pins I have, have different heads, some appear to be made like a rivet (The pin being held, and a head formed by hitting it) some look like they are rolled( the head appears to be formed by rolling the material back on its self, and you can see in under the head from the pin end, around the pin circumference)
I believe you are headed in the right direction, with your welded head option.
If your run of English/Canadian track is a success, do you intend to do the Aust. pattern track?
I have just looked at an Australian manual, and it states the track links are made of malleable iron.
The track displacement is 5/8" not 3/8" as I previously stated.