With the popularity of aftermarket carburetors and fuel injection, the supply of Quadrajet tuning parts has dramatically shrunk. It is impossible to exchange early metering rods with late metering rods. Only truck rods (M-series) are now available new so all other series rods must be sourced from the used market.
Since the APT limits how far the primary metering rods can be pulled down in the jets, small changes to the part-throttle fuel mixture may easily be made. Many passenger car metering rods (ie, with a 0.026" power tip) have a 0.006" taper. That means, for example, that the part-throttle cruising fuel mixture of my 52K may be enriched from 0.052" to 0.046", if you can't get your part-throttle fuel mixture correct within the 0.006" taper of your metering rods, you'll have to source a better set of rods or switch to M-series rods.
M-series (0.036" power tip) and P-series (0.026" power tip) have a 0.012" taper. If I were to replace my K-series rod with a M-series rod, I would need to go up 4 jets (from #73 to #77) to compensate for the larger power tip diameter. Since the primary jet is larger, I would need a larger diameter metering rod as well but only the following Edelbrock M-series rods are now available: 0.048" / PN1941, 0.050" / PN 1943, 0.052" / PN 1945, 0.054" / PN 1947. If my 52K metering rod was adjusted the bottom of its travel (ie a diameter of 0.046") and I needed more enrichment (252.3 E-5 in² flow area), the best M-series rod would be the 52M with a leanest flow area of 253.3 E-5 in².