Are 340’s rare

There are 340s out there tweaker fast back that will blow the doors off your 414 CID Engine A-body. 340s have a rep for beating big cubed engines and that includes your 414 CID Fish
If you really want a STROKER. Build a 48 degree SIAMESED 340 with 4.25 stroke by 4.20 Bore with W-2 Heads that flow 350+ that have 2.19 intake valves a very high lift solid roller camshaft and a 4 speed or better tranny. The standard will overtake the automatic just before the finish line. The standard 340 will not even come close to touching the 472 CID engine.
In a light body car you will run in the 8 seconds if your compression is high enough that is. I know, been there, done that.