Convert Thunderbird from 3/4 rig to ful

[Brion Toss]

Hello,
This could be a huge can of variable, the kind that lead to mast failures ó I've seen more than one resulting from exactly this plan. Your rig is designed for certain forces. I don't know precisely what those forces are, as it sounds like you don't have a standard stick, but you need to know, before moving ahead. If we were doing this, we'd design a rig that could do what you want, and then see how it might compare with the changes you have in mind. But I can tell you now that those new uppers would, at the very least, put more compression load on that mast, a lot more. And that without a jibstay, which would add even more compression, you will likely have an unstable masthead.
It is possible that your changes could work, but please do the design work first.
Fair leads,
Brion Toss