Make it a '68 coupe and im in the EXACT same dilemma. Mine ran a 390 once upon a time, but now runs a "D" code 289 . Considering it WAS a 390, and that the car hasnt been used as a donor, most of the stuff required to use a big block should be there, so very little modification, if any, would be required. However, you would need 390 motor mounts, which i believe are very hard to find. I dont know exact details of the "S" code engine, but it is the exact same as the GT 390 engine. Cant help on the manual-auto-manual as mine always ran an auto, so i havent looked into that.
Oh by the way, the 390 out of a truck you have, i have *heard* that the compression ratio could be different, but i dont know what the he!! that means /forums/images/icons/laugh.gif
Gary
Oh by the way, the 390 out of a truck you have, i have *heard* that the compression ratio could be different, but i dont know what the he!! that means /forums/images/icons/laugh.gif
Gary