Need a plan - component selection for restoring 68 340 that runs on 91 oct

That's about right
I would not use FT pistons, but that's me I'd get custom pistons made or put a stroker in it but... just plan on working on your ignition curve
I built a one of a kind 383 4 seed Superbird with a 440 crank, 440 rods and custom Arias step quench pistons- custom crower cam- same duration as 440 HP cam but lots more lift 550 I think and exhaust was not stretched as Big as 440 HP is way to long on the closing ramps- as is the the 340 we ran dual Isky springs and broke in on the outers
maybe more of a PITA than you want to do
looked stock but very stealth I can post a pic of the piston we made it true 9.5
same problem- nothing off the shelf that works
FT pistons and open chamber heads work best with low gears where you are not working in the 2500-3500 rpm range- ie brackets or a heads up stock class and that's not you
we did what are now Dura Bond powdered metal exhaust seats- larger intake and exhaust valves Back then they were mfg in Washington State till Dura Bond bought them out
If you must do FT piston check you X heads for spark plug threads and polish the combustion chambers, do NOT polish the Intakes
see the DIY porting guidelines at the top of the forum valve job makes a big difference 45 60 about as wide as the 45 then 75 and 82 (will not be much if any) than blend the bottom cut into the bowl you will want a top cut or two and open the chamber around the intake back cut the intakes
stock intake works up to about the same rpm as where the exhaust starts to get restrictive so no use building for more rpm than that
so you end up building for a big fat flat "engine masters" type torque curve

Let AJ do the Wallace calculator
you will have to run half point less compression with the FT/X combination than with a tight step quench piston or closed chamber heads
can you get away with 308 heads? guess not
ask the forum about porting the exhaust- that's where the stock heads hurt the worst
323 gears- do not go nuts on the duration unless this is a freeway/ mountain road kind of build
you sticking with Hyd lifters? Voodoo is a safe choice I'll have to look up that 703
if you have great attention to detail the Howards have more potential
Iron rockers will work fine as I have posted I'd run Magnum lifters and oil through the pushrods hollow pushrods to oil the rocker balls
The iron rockers should have a squirt hole but it can miss with the later longer adjusters
let me see if I have any small block
if not rocker arm specialists in Redding is best people to work with
you can run stock rockers but getting the pushrod length exactly right is a PITA
If you do go roller tip rockers plan on the B3 fix and go 1.6 on the intakes (and maybe one size shorter cam depending on what CR you come up wit)
should be a nice build