As mentioned, you possibly have an internal balance and external balance component mix issue. A forged steel crankshafts big block engine has a neutral balance torque converter or flywheel and neutral balance vibration damper, or harmonic balancer, except for 1970-1972 440 6-barrel and many 1970-1974 440 4-barrel high performance engines which had a weighted vibration damper. A cast iron crankshaft big block engine has a weighted torque converter or offset lightened flywheel and a weighted vibration damper, or harmonic balancer. Factory flex plates are all neutral balance. The torque converter weights for a cast iron crankshaft between the 400 and 440 is the same. Pre-1977 will have a weight on each side of the drain plug. A 1977-1978 will have a single "butterfly" weight and no drain plug. Weights can be added or removed on torque converters.
Forged steel crankshaft harmonic balancer and cast iron crankshaft weighted harmonic balancer:
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Forged steel crankshaft torque converter and cast iron crankshaft weighted torque converter:
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The 1972 400 could have a forged steel or cast iron crankshaft depending on application. The crankshaft for the 400 was a carryover from similar 1971 383 applications. For the following model years, the forged steel crankshaft use in the 400 was phased out. A similar introduction and usage for cast iron crankshafts began in the 1973 model year for 440 engines. The last use of 400 forged steel crankshafts in passenger cars was 1974, typically in manual transmission applications, but were also used in engine numbers 4T400-1-03 through 4T400-1-08, which were two barrel automatic transmission engines. The last use of 440 forged steel crankshafts in passenger cars was 1975. Forged steel crankshafts continued in some Dodge truck applications through 1978, also typically manual transmissions, but most were cast iron.
Directly verify the crankshaft in the 400 if possible. If the 400 has a cast iron crankshaft with a weighted harmonic balancer, it should also have a weighted torque converter. Conversely, if the 400 has a forged steel crankshaft and non-weighted balancer, it should not have a weighted converter.
Your 1966 383 torque converter is a neutral balance converter and will work if the 400 is a forged steel crankshaft neutral internal balance engine. Conversely, for a cast iron crankshaft setup, 400 weights could be added. Aftermarket flex plates, such as from B&M, are also available for using neutral balance torque converters with external balance engines.
Regarding 1972 400 crankshaft usage, reference NOTE 2 in the following from the 1972 Passenger Car Parts Catalog regarding 400 engine crankshafts:
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Reference the application section for crankshafts used in the 400 in 1972. The forged steel crankshaft # 2268114 is from the 383.
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and buy new?
