I can only reiterate that there is no evidence at all for the existence of an Me 323Z, even as a drawing board project. If any moderators are reading this, discussion of it belongs in the 'Theoretical, Fake and Generic Projects' thread.
There is the barest shred of evidence to support Thomas Hitchcock's Ju 290 E, as outlined in Monogram Close-Up 3. This comes from the well-known report A.I.2(G) Report No. 2383 German Aircraft: New and Projected Types by Horace King, dated January 1946.
This gives only the most minimal description of the Ju 290 E as "a night bomber likewise based on the C and having a large bomb bay under the fuselage" - see below. Whether Hitchcock extrapolated his description and drawing from that, or whether he had some independent information on it is unknown.
The closest I can get to it is some Junkers graphs dated June 1944 showing what the performance of the Ju 290 might have been like with four Jumo 222 E/Fs (earlier graphs, from March 1944, show projected performance with Jumo 222 A/Bs). These graphs don't mention a C, D or E designation.
Incidentally, it would seem that there were numerous other Ju 290 variants that have never been reported on before as far as I'm aware, such as a six-engined Ju 290 (small wing area) and two different wings for the Ju 390. The last variant I could find evidence on, from February 1945, was a Ju 290 A (sub-type unspecified) fitted out to carry either two or three Re 4 piloted flying bombs.
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