265 hemi straight six

Pretty sure Slant Auto Trans bolts up

Nope. Some Hemi-6 blocks, not all of them, will accept an A/LA-pattern bellhousing, because the A904LA Torqueflite was used in 6-cylinder Valiant VIPs, instead of the crude BW35 used in less-ritzy Valiants. But TTBOMK there aren't any Hemi-6 blocks that will accept a Slant-6 bellhousing.