The C-354 and the C24-X chokes are about the same size and shape and both mount on 2 3/8" spaced holes. Here are the differences.
1. The C-354 is a 1.5 Henry choke with an internal resistance of about 60 to 65 ohms. The C24-X is a 1.0 Henry choke with an internal resistance of about 50 ohms.
2. The C-354 choke is rated at 200 milliamps current flow while the C24-X choke is rated slightly higher at 240 milliamps current flow.
The original Dynaco ST-70 and the VTA ST-70 use a single choke. The VTA ST-120 and the VTA M-125 monoblocks use TWO C24-X chokes for a 480 milliamp rating.
Both chokes will work well in any Dynaco amp and either choke rarely goes bad. The original C-354 as provided by Dynaco tended to run hot and many original C-354 chokes would leak out a gooey brown substance that would accummulate on the bottom plate directly under the choke. I have switched over to the C24-X choke on all VTA tube amp kits strictly for its higher current rating. Changing out the chokes in your particular amp will have no effect on the sound of the amp.
IMHO > Those with original ST-70's should probably open the amp up and check on the bottom plate under the choke for the "Dynaco brown choke goo". If any brown goo is there I would replace the choke with a (Triad Magnetics) C24-X choke.