305 4bbl? 2bbl? which application do you have? Or want?
Sorry to be a dlck about it, but Malibu with a V8 covers a lot of ground.
But, with limited info to zero in on it,
JimmyCamino is right, the 1254180 coded HU (79 lists same part number but HLL code. WTF?) seems to be the one for the 4 bbl. Used pretty much for Malibu and Monte Carlo or just about any A/G body from 78-88 with a 305 4 bbl. Makes sense. My information puts it at 23.2" long which is right in line with your middle length of 23.5 that you've found.
For the 1254180, the ATP Y-205 and Pioneer CA-8475 seems to be the aftermarket replacement for the GM original.
305U shows a p/n of 1254176 (2 bbl code ON). No length I can find. Superseded to 25509266 then to 25500209. Aftermarket ATP Y-211, or Pioneer CA-8474 Apparently it fits V6 as well?
Best I can do for now.