The Ju-390's problem was that, thanks to the very poor German supercharging and relatively low achieved head pressure on their engines, it was closer in performance to a C-46 or DC-4 than a B-29 or B-36.
Without a solid 2,000HP per engine -at cruise height-, you could not make a 30-50Kft transit 'beyond the weather' and any kind of headwind or icing condition is gonna start taking massive amounts off the topend of your target-return radius.
This does not necessarily mean that the aircraft could not make the trip, because as men as famous as Lindberg have proven time and again, it is easily possible to use a combination of power settings, cowl flap (to maintain fuel and oil temps at suboptimal rpms), lift flap and base mixture to 'fly up' the profile from a relatively sluggardly start to almost twice the height:range nominally achievable (with a P-38 in Lindbergh's case) in the flight manual by end of radius.
Where the Ju-390 likely would have made good it's supposed payload shortcoming was in the use of 'deichsel schlepp' or literally drag tanks. What we would call towed fuel tanks. Just as with a horse, an aircraft can -pull- twice or more what it can -carry- and by exploiting existing technology base (V-1 missiles without engines or warhead) the SG-5041 (sonder gerat or 'special device') would have supplied upwards of 1,200l or more of fuel. Toss in air to air refueling using Ju-290 crews specifically trained in that role for no other explicable reason and you can easily make NYC, even with a rather unwieldy Plutonium Weapon. Probably even come back.
And that's what this ultimately comes down to. There is far too much secondary evidence (1,500 tons of Uranium gone and never accounted for from prewar held European stocks, 1,000s of troops and large runways built in Norway, late-war AAR testing, massive exchanges of key materials like Mercury, maintenance of the Ju-390 itself long after other aircraft like the Me-264 had been abandoned, the pell mell rush by Patton to secure the last 'bell' from Pilsen, the coverup as to how many Ju-390s actually were built etc.) to ignore the reality that the Germans were likely a lot closer to an atomic weapon than we would like to publically admit.
Because a nuke on London and a nuke on NYC, along with a flurry of battlefield weapon releases, is effectively the only hope the Germans had and the _only reason_ Hitler would have attacked the U.S. and Britain in The West during Watch On The Rhein while depleting Berlin defenses to hold onto the Czech corridor from which he hoped to source his weapons out of the Skoda Works.
Once you accept that this is exactly what it seems like, a play for time sufficient to _stay in the game_ with nukes, the entirety of the Ju-390's purpose and the covert flights to NYC become logical proof of concept tests to provide the '5 minutes after midnight' rescue that Hitler demanded his troops continue fighting for, long after the situation became tactically hopeless. As long as we 'never know' about the Allied Myth of nuclear supremacy; it is probable that the details of the Ju-390 missions will always remain disputed if not actively aspersioned by the 'experts'.
CJ