What engine and transmission does my 79 Xjs have ?
 

So I bought this 79 Xjs yesterday and I know these vehicles get the American 350 engine swapped in there . What my question is , how would I know what year Chevy engine I have and if the transmission is American as well ? Is it possible that the engine can be American and the tranny European or does the engine have to match the tranny by being American as well. Thank you for your time folks !

I think some engine and trans identifiers are required, like serial / casting numbers and photos.
At the moment you are asking how long is piece of string.

10ft. My first guess is the string is 10ft. long

Your car would have originally been built with a GM TH400 transmision. It was a unique variant of this transmission for use behind the Jaguar V12. When the conversion to the Chevy engine was performed the original transmission my have been retained....or not. If you see an adapter plate between the engine and transmission it would suggest the original transmission was retained.

Entire books have been written regarding identifying Chevy engines. There were hundreds of variants, ID numbers, etc. Begin by writing down any casting or stamped numbers you see, including the numbers (if any) on the flat pad at the front of the engine, just below the RH cylinder head.

Cheers
DD