In reply to:
You underestimate my cheapness.
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Okay then... just be SURE that you VERY CAREFULLY mark or somehow indicate which lifter came from which hole. The lifters mate to the cam in the break-in process. Improper break-in will wipe a lobe or even the entire cam, and I would guess that swapping lifters around could easily do the same.
As for the pushrods... is there something wrong with the 258's? I don't know if there is any difference, but I would lean toward staying with the 258's just in case there was any difference in height. If you really want to go with the 4.0's, at least put one side by side with the 258's and see if you can notice any difference. Since it is a hydraulic train, I doubt that a difference smaller than the human eye could detect would matter. It's going to build up "x" amount of hydraulic pressure in the lifters anyway. Like I said, I KNOW from personal experience that the 258's pushrods work fine with a 258 short block and a 4.0 head.
Haven't played any with the Megasquirt. It may be fully tuneable, but that doesn't make figuring out what to tune it to any easier. I'd do some hefty research here. Of course a lot depends on your own mechanical prowess and resources as well. I found out the hard way that tuning an engine is a complex formula of timing, throttle, mixture, combustion chamber shape (which you are changing), and atmospheric conditions. Wound up doing a fair amount of fiddling with my carb and distributor to get things as good as they are - which still may not be perfect, but is pretty durn good.