1976 Valiant is almost certainly a 318 if it's a V8. The 340 was long gone by 1976 and the 360 was pretty rare (though there were Valiant 360 powered cop cars).
If you swap to a 318, you need to address two issues:
- The front of the motor is probably set up for a radiator with at least on of the outlets swapped to the other side. You either need a late model radiator or to transplant the front cover and accessories from the 273 to the 318.
- The pilot hole on the torque converter will be wrong. I can't recall if the 1967 was larger or smaller than the 1968 up.
If you swap to a 340 then you add one more issue:
- One motor mount is different, believe it's the left side one.
If you swap to a 360 then there is still one more issue:
- The 360 requires a weighted torque converter since the 360 is externally balanced. (or a weighted flex plate)
It's not a true bolt in for any of these, but pretty easy as swaps go. All engines (273, 318, 340 and 360) have the same external dimensions and are close in weight.