This is EXACTLY backwards. Given the same amount of force on the pedal (same pedal effort) and same booster assist:
Larger master cylinder bore = LOWER pressure. (but higher volume) This is why the larger-bore master cylinders ALSO had a bigger power booster.
Smaller master cylinder bore = HIGHER pressure (but reduced volume; which is why the low-drag calipers need the surge of low-pressure, high-volume fluid at the beginning of brake application.)
Replacing a step-bore master cylinder with a "larger" straight-bore (while retaining the low-drag calipers and the same power booster) is a TERRIBLE idea. If you're going to scrap the low-drag calipers, then a straight-bore master can work. But if you're complaining about low or squishy pedal, and poor stopping...it's not an inherent problem with the low-drag calipers or the step-bore master. There's a FAULT somewhere, potentially more than one--and you don't need to redesign the front braking system to fix it, you just need to find and correct the fault(s).
Three common suspects: Faulty ABS valving; faulty combination valve, air trapped in the ABS, the master, or ANYWHERE ELSE in the system. If, for example, the master cylinder is tipped "up" in front, they'll often trap an air bubble that gallons of brake fluid won't remove. If the valves in the ABS unit don't open and close, and seal properly as they're supposed to, the pedal is going to be affected depending on what exactly has failed. Could be high pedal. Could be low pedal. Could be firm pedal. Could be squishy. And in any event, braking power suffers.
The REAR brakes on the 1500s tend to have the leading/trailing shoe design. I've heard rumors about why; but no hard evidence. The rear brakes suck on 1500s. If you don't use the park brake, the rears don't adjust properly and the pedal goes low, but firm. The front shoe wears faster than the rear shoe. And there's little self-energizing.
Let's not forget that some of the early GMT400s had JB3 brakes. Of course they stopped poorly. My own '88 is getting JB5 brakes from an extended-cab; in fact when I stop typing here, I might actually go outside and continue that conversion. The change involves a front caliper with larger piston (but still low-drag calipers), a larger step-bore "QTU" master cylinder, and a larger power booster.
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