I've seen the "400" badges used on both a 400 Small block, as well as 396/402 Big blocks.
IF I remember right, the Big Blocks have the "400" in Red letters.

 

The 400 emblem, indicated most likely 400 Small Block, (as seen in the picture I attached of the 1971 Impala on ebay), but the only way to know for sure is to lift up the hood. As they say "Looks can be decieving".

-Brian

 

I believe the use of the emblem was dependant upon the 'status' of the engine, i.e. whether it was a standard engine or an optional engine. The '71 Caprice had a 400 small block as standard and would not carry the badge, but the same engine in an Impala or Bel Air would have the badge. The 400 badge was used in all cases however (Caprice included) if the optional Turbo-Jet 400 (402) was ordered.
If stock, the small block 400 had a single RHS tailpipe on '71 Big Cars, while the Turbo-Jet 400 (402) came with duals. All big car wagons from '71 - '73 used single exhaust systems regardless.

 

thanks for the info .i am not sure about the tail pipes i need to get a better look . i do know that the 400 imblem is red. kinda funny looking real thick . i think i will go ahead and get the car . that is if i can ever find some one at home . thanks again mikess396