Will a th350 fit 455 engine?

The short answer is yes, however it depends on the exact variant and year of the 455 engine in question as well as the variant of TH350 transmission.

The TH350 transmission was widely used with various GM vehicles with several variants of the 455 engine, particularly in Oldsmobile and Buick vehicles from the 1970s and early 1980s. As long as the transmission and the engine have compatible mating surface, it is generally possible to fit a TH350 transmission to a 455 engine from the same family and era, but it's advisable to consult with a knowledgeable mechanic or refer to vehicle-specific technical documentation for the exact pairing you are considering.