This is a good example of why if someone brings me a motor with the intent of me making a few changes to increase the power output....... it gets put on the dyno before being taken apart.

This does two things(for a drag application).....
-establishes a baseline number to work from.
-helps determine if the “car” is running the et’s it should for the power the motor currently makes.

I’ve seen it go both ways before.
-The car was slow, but the motor was doing exactly what it should...... so no changes were made to the motor......... and the customer diligently worked on the car and ultimately picked up over .5 ET without making any more power.
-The car was slow...... and the motor was down a solid 50hp from what I was getting from very similar combos.

Of course, then there’s the situation where someone buys a motor that came with a dyno sheet, and the car hasn’t been running et’s or speeds representative of the dyno numbers are....... and they have spent thousands on fuel system upgrades, trans parts, converters, maybe some gears, etc....... all to no avail.
Then we put it on the dyno and it’s down 50, 75, 100hp from what the dyno sheet that came with the motor shows......... and the car is going exactly as fast as it should, based on our numbers.
It’s this scenario that lead me to start testing before any modifications begin....... to make sure everyone is on the same page right from the start.

The OP’s car is running way way off from what it should.
It shouldn’t be that hard to come up with a realistic horsepower estimate for that combination of parts.
If the motor makes within +/- 30hp of that figure....... you can leave the motor combo alone........ it’s not your problem.
On the other hand, if it’s down 100hp....... better get it apart and start looking.

Obviously, you’d like to test at a dyno service that has a good history of cars running times/speeds that are in line with what’s expected based on the dyno numbers.