Option D None of the above.... Build a 408 Stroker using a later 360 roller block.... And stuff the 340 under the work bench...
And here's why...
340..... If you when with the original 340 it would be all stock, while stock 340's do run good you'll be bored... And if you decide to beat it & it got hurt or honestly even if you didn't beat it & something went wrong & the motor got hurt......
426.... You've got allot of nice parts but getting the valve train properly sorted is gonna be more trouble than most folks realize & even if/when you've got it all sorted how much would you really drive it? 99% of all the Hemi's I've seen in the past 30 years spent more time sitting with the hood up with the owner sitting in his lawn chair doing the "Look at my HEMI" thing... Ok so your not another pop the hood/look at me type of guy, then whats the point of the Hemi, I'll admit they have an impressive top end charge but a stout 408/426/440 stroker small block can run with it while pulling down a heck of allot better mileage...
Oh, also to go Hemi there's the trans, exhaust, you'll want a Dana, brackets, pulleys, mounts, K member,Radiator... It's all doa-ble but the stroker small block bolts right in....
BTW I owned an original 71 HemiChallenger so I kinda know a little about this...