Decals cost about $6 dollars. The 375 horse 396 differed from the 325 and the 350 horse 396 in the heads (only the 375 horse 396 received the HiPerf rectangular port head that is a dog on the street, the other two 396 engines used the much more practical and functional PASS oval port heads). The 375 horse 396 also used a solid tappet rough idling race cam designed for high RPM operation (NASCAR racing). And finally the 375 horse 396 has an aluminum high rise intake manifold (that saves 23 pounds) topped off by a 780 cfm Holley vacuum secondary carburetor. (If this 375 horse 396 was originally installed in a Corvette, it will also have a dual point distributor with a mechanical tach access on the side of the distributor).
The factory learned a bitter and costly lesson about solid tappet high RPM big inch motors with the 409. Namely any vehicle with a solid tappet cam could rev high enough to self destruct; or more frequently to take out all of the accessories on the front of the engine. So this also includes the solid tappet 350-360 horse 327 and the 283 horse 283 motor.
Are you aware that your stock riveted steel fan will shed it's blades above 4800 RPM?
Further air conditioning compressors will blow out every seal in the compressor at about the same engine speed. Even your water pump and alternator are nor equipped with bearings that can spin very high and will fail if pushed.
Chevrolet discovered these issues on all of their solid tappet cars and either restricted the availability of things like A/C (you can not get A/C on any engine equipped with a solid tappet); or to modify and reinforce the parts being stressed. Which is why the Corvette has it's own dual point distributor, short water pump, even after the long pumps were introduced in 1969, and a special high RPM alternator with special bearings and special alternator fan. (the Corvette short water pump also used a larger diameter shaft with sealed, greased for life roller bearings, instead of open ball bearings that need lubricant in the coolant to keep the bearings from burning up).
Because an Impala is a large, that is to say heavy car, combined with the fact that the 375 horse 396 had no bottom end torque to speak of, it wasn't installed in the Impala. The clacking of a solid lifter engine probably would also restrict a lot of buyers who would complain about the car's motor sounding like a sewing machine. Finally because the cam was so aggressive, it would require a manual transmission with the Impala to get the car rolling (this is because a loose converter had not been invented yet; which is why all the solid tappet 1967-'69 Z/28 302 motors where all bolted to a Muncie four speed and required set of 4.11 or 4.56 rear gears in a light Camaro body).
Check to see if your 375 horse 396 car has a single bump on the head to identify it as being a HiPerf rectangular port head (here is an example of the casting mark)
The difference in port size is hidden from view with the intake in place which is why you have to look for the that little casting identifier. But they are very different:
The intake difference between aluminum and cast iron is obvious, but the factory manifolds all have a Winters Foundry casting mark; not found on aftermarket castings with the name Edelbrock or Offenhauser ground off:
Same for the Holley carb; the 350 horse 396 used a Holley with square, single feed fuel bowls (a 4160 style Holley). The 375 horse 396 used the center hung dual feed Cathedral style fuel bowls (a 4150 style Holley). The 325 horse 396 used a Rochester QuadraJet.
The other difference is the 375 had chromed valve covers with spot welded drippers under the valve cover to oil the rocker balls (that were grooved on the 375 horse but smooth on the 325 or 350 horse 396). You should be able to see every spot weld mark in the chrome where the drippers are attached. After market valve covers are just chromed formerly painted 325/350 horse valve covers that did not have the drippers attached; and as such have no spot welding marks visible (just like the ones on the 450 horse 454 that some one claims is all original on the high performance head photo above). I can probably list ten or more differences between the motors but you have to be able to take it apart to see them.