Hi All, I have built 2 strippers with a internal strongback and heard that an external strongback is superior to the internal.

I've learned that 'superior' can be fightin' words online! ;-)
Let's not get involved in that!

I do prefer the 'traditional' external strongback - I started building with the Hazen and Ted Moores books, and later adopted some of Jay Babina's ideas- and they all use the external strongback, IIRC.

I use a simple 'T'-section strongback made from 2x6 lumber- 16 footers. Legs are provided by a couple of metal sawhorse brackets (which grip the 'spine' of the 'T') and 2x4s. I now use shelf brackets for attaching the forms to the strongback, but that might not be 'rugged' enough for most folks. Previously, I used the 'bolt-on' technique from the Moores books, and it works well.