Cliffs Carburetors, https://cliffshighperformance.com/product/quadrajet-power-piston-springs It will take some trial runs to get the correct one for your truck. Stock may not be right since the motor is a trans plant. Vacuum controls the power piston and metering rods, the spring fights the vacuum. Stiff springs can cause richer metering, light springs offer leaner mixtures, both will depend on how much vacuum the engine pulls and how you drive. Ideally you want the metering rods to pull out of the jet just enough to richen the mixture when the engine calls for it, not before. This set up is similar to a Holley power valve setup, both add fuel when the vacuum reaches a certain level. Holley made it easy and marked the power valves at the opening vacuum, 3.5 4.5 etc... Rochester makes us work for it.

1937 Chevrolet Inline-powered Streetrod