I suspect that the neck and body are simply from different guitars. But it's definitely not clear cut, and there are several layers of non-obvious odd going on here, as often with Greco Fender copies.
Like you said, the 800 had a pre-CBS headstock in 1978. That feature came in on 600s in 1979, and so would have been present on a Nov-Dec 1978 guitar of that model, but that one is April. And anyway, the MH803 diamond tuners are a 1977 SE800 feature, the '78 version having the Kluson-style SD81-05Ms. Like you said, a rosewood fretboard neck was not an SE800 catalog option... which doesn't mean that none were made. 50's style guitars with rosewood boards is one of the more common Greco SE anomalies.
But there actually is one more possible source of a 1978 neck with a rosewood slab, pre-CBS headstock and MH803s: the SE500V, a 1978-only colour option (a stained violin burst). For some unfathomable reason, that specific model in that specific finish was paired with a pre-CBS headstock. All other SE500 variants had a large headstock, as did the V-finish version of the '78 SE600.
Yeah, go figure.
It's well hidden away, though. No pics, it's just mentioned in brackets in the colour options listing at the end of the specs for the SE500. According to Google Lens:
Gotta love "Old Head". Also note "Rose fingerboard". The catalog copy also specifies MH803 tuners.
This one's in my collection. Not counting the body finish, it's a pretty good fit with yours.
The only other catalog models I can think of with a small headstock and MH803s are three guitars (SE1200, -800 and -700) from the limited edition Project Series, but I can't recall ever seeing one of those with a rosewood fretboard (which doesn't mean etc...). And if the neck plate is original to the neck, it's not off a Project Series guitar, it would have had the model number engraved. Inconclusive, to say the least.
As for the body, that could well be from a 1978 SE500. Not many details to go on, but it looks like it has the narrow (around 10 mm) bridge saddles of that model and year, and the pickguard's correct as well. But if the latter is original, it's a bad fit with the neck.
For some strange reason, Greco never got the 1959 transition from single-ply 8 screw to laminated 11-screw guards right. Instead, they put single-ply 'guards on all models with pre-CBS headstocks and laminate guards on everything with CBS headstock (except, of course, the above-mentioned SE500V...). As far as I've seen, they kept this up right until Fender copy production ended in April 1982. So by and large, the body is a poor fit for the neck.
But (and here's the thing): even though a parts guitar is maybe the likeliest explanation, it's
by no means certain. One of the things that make Grecos fun to be around is that they offered made-to-order builds to end-users and retailers from as early as 1972, ie what we would call a "custom shop" today. They could do anything from ground-up builds from client sketches to "minor change" jobs, in which standard features were combined in non-standard ways. Which could well be the case with this guitar.
But this can only be determined by forensic study or at least inside-out detail pics. Or at least possibly determined, I have a small menagerie of off-catalog Greco SE oddballs (I havent counted in a while but 15-20 or so?) and only a single one of them is clearly marked "Order Made". A lot of them are small batches made for distributors and/or retailers wanting something special to offer their customers and can sometimed be traced through store ads in old Japanese guitar mags. But these tend to be slightly more spectacular than this one (all-mahogany construction, bound fretboard, metallic finishes, Mustang-style racing stripes etc).
If the OP guitar is indeed a factory original, I would assume it's a single end-user order under the "minor change" scheme. However, to make absolutely sure about this is almost impossible. It could of course have some rare "order made" marking inside, but if all parts are Greco/MIJ and in period there is at least reason to suspect it. A good set of inside detail pics would help, original pots and pickguard should be date coded etc.
Either way, nice-looking guitar, and Greco SE necks are almost always excellent.