Well here it is after all that blood sweat and tears! I used MK 319 BS pop rivets for ALL the underside and tab ends. Vans in a RVator article give authority to use these in place of AN426AD3-3 to 4 solid rivets. In a seperate email Vans said I could use them on the trim tab in place of the CS4-4's. I couldn't find too much about these rivets so after some experiments I found the following:
1. If you know you are going to use these in advance drill/match drill to 7/64" and dimple with a 3/32" die. The die enlarges the 7/64" hole and permits the MK 319 BS to fit.
2. If you DECIDE AFTER you've drilled/match drilled to 3/32" and dimpled with a 3/32" die then drill out witha #30 drill to permit the rivet to fit.
3. The tab ends were countersunk using the debur tool and a solid rivet to check for fit.
4. The trim tab horns (E-717 and 8 or electric trim equivilant[shown]) are not riveted together and could leave a trap for moisture. In an email Vans recommended using proseal but as I had none available at this stage of the build I used epoxy and jointing compound (JC5A) undeneath.
