Can anyone confirm that the postcard was a standard, 5 by 7 inch? If yes, then the distance between the two crosshairs symbols is 3.3 inch!

This is precisely what we found in Z32 and, translated to 21.1 miles according to the Mount Diablo map scale, indeed lands us again in Paradise Drive, Fremont. If so, Zodiac didn’t trust us to solve Z32, nor understand the Halloween communication, and just provided the Vallejo Mill location once again in this Sierra postcard.

I’m pretty sure that I got the dimensions right, as they render the printed stamp a standard 1.2 by 0.9 inch, but I can’t find a picture of the postcard with a ruler for definiteness.

Edit: Other elements in the postcard may pertain to Fremont. From Wikipedia: "Lincoln appointed Frémont Union Army Major General … From an area near what later became Virginia City, Frémont turned west into the cold and snowy Sierra Nevada, becoming one of the first Americans to see Lake Tahoe …. Fremont, California (the largest city that bears his name)". Zodiac may have taken inspiration from a similar text when looking to introduce ‘Fremont’ hints. Of course, the related postcard elements may contain a different message, instead or in addition (as Zodiac liked) to Fremont.

Edit 2: In addition to the non-enciphered bomb location specified by the relative positions of the crosshairs symbols (non-ambiguous: uniquely viable location, exactly repeats Z32, consistent with Halloween communication) and the clues provided by the word content (ambiguous, perhaps Fremont related), there is almost certainly a concealed cipher in this postcard. One can tell by the awkward choice of words, their unnatural arrangement and length pattern, irregularities in letter capitalization, etc. The equality sign and other anomalies in the address line, "Paul averly = ChronicL?e", suggest a (10 or 10X10 long) key. This non-homophonic cipher is probably solvable, given the gradually easier and more hint-supplemented puzzles provided by Zodiac since Z340, assuming a stand-alone postcard with no missing pieces. However, if the message is another bomb reference, then it is probably redundant at this point.

Edit 3: There is a postcard image with a ruler here. If this ruler is correct and one inch in total, then the crosshairs symbols are only 2.45 inch apart, but the postcard is then too small to convey the full 3.3 inch at the necessary angle (and the full height is below the 3.5 inch minimum for a postcard), forcing Zodiac to rescale to a standard postcard/stamp – that would be typical, safer than directly giving the distance, and may also explain the stamp reference in the scalloped edges of the postcard. The angle is of course unchanged.

Indeed, there is a concealed cipher in this postcard. I think I understand at least most of it. I could not find this cipher mentioned anywhere on the web, and I’m not sure why.

As pointed out above, the equality sign and symmetry give away the address on the envelope as a ten-symbol key:
Paulaverly=ChronicLke.
Zodiac thus continued his Halloween-card spirit by further reducing Avery’s name from a clue to half a key… He added the redundant “l” and “k” to get ten symbols on each side, so we should expect a ten-symbol message. Zodiac had to capitalize the “L” to distinguish it from the “k”, and kept the “a” deliberately in lowercase to indicate part of a key.

This kind of equality key strongly suggests a classical shifted-alphabet cipher, discussed in many amateur books, where the initials of the subsequent text form the coded message. The first key, “P=C”, usually translates the initial {p,q,r,…} of the first text word into {c,d,e,…} (respectively and cyclically); the second key, “a=h”, similarly translates the initial of the second word, and so on. An additional small shift s is sometimes introduced; for example, an s=1 shift would effectively replace “Paul…” in the key by “Qbvm…”. The shift s can be changed mid-sentence, especially if signaled by a code word or a number.

The key seems obvious to me, but where is the message? The bottom-right text, “pass LAKE TAHOE areas”, sounds unnatural and very likely to end the message, whereas the preceding C in “viCtim” was capitalized to draw attention (typical of Zodiac), so together we have (…,c,p,l,t,a). The number 12 here may indicate s=1 switching to s=2 or vice versa (but probably not s=12, which seems uncomfortably large). Where does the message begin? We need five more ordered words, and their choice is ambiguous. “Peek through the pines” is in quotations, so probably unrelated. Starting at “around” (the first word you see when first flipping the card) is elegant as it suggests a reading order (and perhaps indicates additional irregularities or an inverted final message): “in the snow Sierra Club”. Translating the resulting (i,t,s,s,c,c,p,l,t,a) with s=2 changing to s=1 mid-sentence gives "Fremont nyt" (inverted: the more robust "cplta" ending of the postcard yields the "Fremo"). I think Zodiac indeed played ‘around’, and we actually need “the in snow Sierra Club”, which yields “Fremont NNE”, pointing at the bomb location we already know is north-north-east of Fremont.

We anticipated “Fremont”, which is quite robustly obtained, supporting the solution. The ending “NNE” seems likely to me (Zodiac writing “the in snow” would be too conspicuous), but perhaps one can find a more likely modification of these three letters or somehow interpret the alternative “nyt”. The solution may seem too forced if you never worked on such ciphers, but note that the ambiguity here is actually very limited: the fixed key is entirely different from e.g. homophonic ciphers, leaving very little flexibility. Indeed, try to generate other meaningful messages by changing the reading order and varying s: you will generally fail to form anything more than an unrelated three-letter word, and even that is not so easy. So again, the bulk of the solution is quite unique.