The stock rockers are ideal for anything but a higher rpm engine. Stud mounted roller rockers are much weaker than OEM stamped steel on pedestal rockers. The stud becomes a notable weak link, constant high rpm is bad for roller rockers(not shaft type). The worst are the 5/16" stud roller rockers made for the stock pedestals. Most aftermarket roller rockers used to be 3/8" studs for cut down pads. Now the 3/8" rockers are uncommon, most good brands all make 7/16" stud rockers now. Those are much stronger and will survive much longer in street engines. So avoid going to roller rockers unless you are willing to spend more time inspecting them once in a while, say once a year.
The pushrod length should be checked for anytime the valve lift changes much, and when different type/size springs or rockers are installed. So what is the lift on that cam, stock is in the .450" range?
The stock Explorer 302's are good to shift at 5000rpm, bone stock. All three of mine have shifted at 5k virtually every time I hold WOT, that's with stock programming, everything. The limitation to rpm is the cam, but the GT40 heads and intake are not big enough to gain much more rpm, given a bigger cam. So I'd start with the 5000rpm shift, and slowly see if there's anything to gain past that.
I haven't played with setting up a new cam on stock heads, which are pedestal mounts. The pushrods basically need to change length as the lift changes, getting shorter slightly as the lift grows. But the geometry is about the rocker geometry, the alignment of those versus the valve tip, and the pushrod. I gather that often higher lift forces the owner to add shims under the rocker base/fulcrum, just a hair, to correct the rocker geometry.
You want the rocker to be equal in angle to both the pushrod, and valve, at half lift. That's the general goal for best geometry, but most descriptions have you concentrate on the valve tip wear pattern. The wear pattern is adjusted to achieve what I described in the first sentence, which results in maximum clearance from the rocker, to the pushrod and valve tip. Without changing anything but the cam and springs, the pushrod will be getting much closer to the rocker arm at maximum lift. So watch those points carefully.
