The blocks are the same as a 4spd block, just drilled differently. The first ones were around 3/73 from memory, cast for F5000 use. Later ones were cast for L34. The first ones drilled in TH pattern will be for the first 308's with TH400, probably actually on HQ's in Engineering. The first production HJ's with 308 TH400 will be circa July-August 1974.
Essentially yes, M41 on a HQ means a 350, but you'll only find it on Statesman, Statesman Deville, LS, GTS and Kingswood sedan (XW8 - GTS 350 sedan). I'm pretty sure all the "specials" identified with 350's in them that aren't supposed to were early body plates (no mechanical options on them).