Good afternoon ,
We are working on a 1955 Mercury 292 with Ford o matic three speed transmission .
This is a well maintained automobile with all new ignition , Holley carb , and new edelbrock fuel pump . 
This car was running great untill it suddenly was not able to accelerate the car forward or backward with out full throttle . really not good .
Feels like it went from 200 hp to about 20 hp wide open , like a trans brake ???
All brakes inspected car stops great .
It feels like the exhaust is blocked up big time but the exhaust is a year old and feels fine .Dual exhaust and flowmasters .
No backfiring , no exhaust blockage , compression test at 120 psi , valves adjusted to .019 .Motor appears new inside valve cover .
Static vacuum cranking test pulled a wopping  TEN  inches of vacuum . Starts right up , SOUNDS great .
The owner did do a BURN OUT on concrete right before this girl changed her personality immediately .
We believe we may have failed a converter or a sprag or welded the clutches ????
Any thoughts on this sudden change in behavior are thanked !!!
Happy and safe travels to all.