slant6 to 318 c.i. small block (similar in size to 273 c.i.). will my A904 trans..

Yes, you will need a later slip-yoke driveshaft. 1967+ may be too long, so 1966 A-body may be your only choice, though shops can easily shorten one, which many here must do when changing to a 8-3/4 rear-end. You also need a 64-66 V-8 steering center-link to clear the oil pan (later ones are too long) and a 64-66 273 oil pan unless you want to strategically rubber mallet a depression in the side of your 318 one to clear the steering linkage. All this is covered in many posts if you google. More issues if you change to a later rear-end w/ BBP wheels and/or front disks.