The correct way to do it is to measure the length and the compression force required to compress them (to see if they are good). I just measure the length. Spring compressor gages are hard to come by.
The lifters are likely going to float, before a stronger spring gives you much if any benefit. You may know something I don't?
I really don't see the benefit in stronger springs. The few more RPM's you'll likely get (top end) aren't worth the extra wear and tear on the valve train IMO. And what good are a few more RPM's at the top of the RPM envelope, where the motor isn't making much power anyway (stock cam and lifters)
From say 20 MPH to around 70-80 MPH, the old 4.0 will hang with many faster vehicles. I drive it in it's envelope and occasionally embarrass somebody.
