7400 was a well-made derailleur, but it had possibly the smallest chain wrap capacity of any derailleur ever made. It was designed for 42/52 rings and a corn cob racing block in the rear. And as correctly posted earlier, it could only handle a 26 tooth cog. Yes, seriously. Finally, in terms of index shifting, it was only compatible with 7400-series shifters; nothing else in the Shimano lineup, then or now.

600EX and other such Shimano derailleurs that predate the Suntour patent expiry, lack the slant parallelogram feature; a critical bit of technology. Hence they produce hesitant, clunky shifting. I've used them all.

Want something functional and somewhat vintage? I suggest Shimano DX, XT or Deore from the late 80's. Derailleur technology hasn't progressed since this.