Ideally you'd want to replace an engine mount with a load cell. You need the cell to take the entire load through it to get a decent result.
You'd also want to make the opposite mount allow at least a full degree of freedom on an axis parallel to the crankshaft (like a spool mount).
Then you could use the distance between the mounts to turn the load cell force into a torque. Record rpm at the same time and you could graph torque and hp too.