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

You got it hank; the balancer.
And I didn't actually mean for you to stroke your 340 to a measly 360. But rather to illustrate the point of what stroking can do for you.
V/P Index Calculation
It's not about the extra cubes. It is about the cam-for-cam stronger low rpm power, that allows you to run less gear,less TC and makes the car more fun in the city.
And face it everybody loves to ride in a convertible, so most of the time you might be several hundred pounds heavier than if you were a notchcie. Therefore, you have some choices;
To get off the line you will need to compensate for those maybe 300 to 500 extra pounds. That means a bigger TC, more gear and or a lower first gear; or more cubes.
But when you hit second gear, that split is gonna hurt you, so you are left with gear or more cubes. Top of second will be close to the speed limit, and to continue briskly will require a bigger cam or more cubes.
Do you see the common theme running thru here?
The 360 is about 40 cubes bigger, just right to compensate for 400 pounds; cam-for-cam and gear for gear.And to build a proper hi-compression 360 will cost you a lil less than a similar 340 in terms of machining, and also in terms of parts. And you can upsize to the next bigger cam without losing as much low-rpm power.

And you never have to worry about blowing up your numbers matching 340. Jus saying.