You can put together an under-40 minute, Dec'66 theoretical single LP pretty easily if you go by Brian's mono mixes and assemblies, the Capital memo and a little fudging around "The Elements". Bearing in mind the 12-track version was unlikely to have link tracks - and almost certainly no crossfading - the actual order of those 12 is up to you. Here's mine just for the sake of argument.
Side One:
Prayer 1.10
Do You Like Worms (with extended music box outro) 3.56
Heroes and Villians ("Cantina" with fade replaced by slow "boys and girls" or other section) 3.03
Wonderful (the usual) 2.00
Child is Father of the Man (Brian mono backing track) 3.03
The Old Master Painter (OMP/YAMS/Barnshine) 1.53
I'm in Great Shape (Workshop/IIGS/Chimes Intro or other section/Barnyard - ie. the other half of the original plan for "Heroes" and additional pieces based on Vosse's comments in Fusion) 3.23
Cabin Essence 3.34
Total: 19 mins
Side Two:
Good Vibrations 3.33
The Elements: Fire/Water 2.30-3.30 (depending what you use)
The Elements: Wind Chimes (full '66 Brian edit) 3.23
The Elements: Vega-Tables (total cheat, as this uses the March sessions. If you really wanted to be a '66 purist you could use the "demo") 3.09
Surf's Up (again, depending on level of purity, CFOTM reprise or no) 4.00 approx
Total time: 17-18 mins
Total album length: 36-37 minutes.
it's also an order that, edited right, is more satisfying as a narrative/musical experience than it might appear in print, and hits most of the major pre-'67 musical beats.
I can't see any reason why a single LP wouldn't have been possible - except, of course, that it wasn't.