idles then dies with slightest throttle up, valve timing?

Yeah, trying to reverse-engineer someone else's build can be challenging. Finding out it was a 750 must've been a surprise bonus. Knowing that it ran before your work helps, could be something simpler then. There were 5 major variations of the BS38 carb, and even though some parts seem to mechanically interchange between these, they really need to all match the year/model type of the carb body. Some of your posts report jetting numbers and such not used on '77 carbs, so need to do a positive ID of all your carb parts. Some help here:

You also asked about backfire and blowing the carbs off. Severe enough, the backfire pressure can collapse brass floats (causing higher fuel level), and rupture diaphrams, especially if the parts are old/corroding/weak. The 'clear tube method' could help confirm the proper fuel level.

I hope it's something simple, often is, but most folks find that 'simple thing' only after going thru everything. Kinda like looking for a dropped screw, they show up where/when you least expect...
Yup, it was something small. Had a condenser wired in where the capacitor needed to be.
Just got back from the first shake down, now begins the tuning process.
Thanks for all the helpfully suggestions and know how; best a luck and god speed.