[valiant67]

Even if you don't stroke it, I'd exchange the 273 for a 360 and dress the 360 with vintage parts like the air cleaner and maybe valve covers.
Dollar for doillar you'll get way more from the 360 and no one will need to know it's a 360.

 

[valiant67]

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.