Shorty Headers

My opinion. If the stock manifolds are a limiting factor in just flow (273 early A is likely a good example) then yes the shorties will help. Shorties (and stock manifolds) are too short to do what header’s actually are suppose to do, scavenging to actually boost power. You need about 28”-36” tubes to do that. Tube size is another hot topic. There is some simple math to size the tubes. If they are shorties and not helping with scavenging go as big as you want. If long tube too large it can cause a decrease in the wave velocity so it does have enough energy to do a good job of starting the backwards wave up the other cylinders tube to effectively scavenge the cylinder. Again simple math with the cylinder volume will size the tube.
Go to bed, Jim! LOL