26K is dam close to 30K! Based on what I've heard from you through the years I wouldn't hesitate to have your builder, "Boss Hog" I belive was the name, build one for me! Sounds like you got a good deal too (well, except for maybe that roller lifter thing a few of us went through, iron or alloy).
I've met and examined several high end alloy engines and the cost is often at or close to $30K. No question rear main seal issue's seem to abound on FE's, iron or alloy. With the Shelby, yup, that issue was identified and corrected. There is NO EXCUSE for that with an iron block though, that's just flat out a screw up on the builders part. I took extra ordinary care putting mine in, it don't leak.
Still, you GOT to have the right builder for an alloy block, you just can't cut any corners.
That last alloy block I checked out was in a converted "Eleanor" 'Stang. Motor built in California, shipped to Hawaii. The car was trailered in for the car show, because of the extreme leak at the rear main seal!!!
That was less than a year ago, he had just a bit over $30K in it....
Oh, I'd go Ebrock heads myself. I'm not sure, but I suspect the Shelby heads ARE Ebrocks with a "name". Nice name, if your willing to pay for it.