You should get a screen that has a kernel name and is blue, at that screen, you should be able to press any key and then select a kernel before the kernel actually starts booting.
Does that work?
The screen comes up, and both the 34 and 42 kernels are displayed, but only for an instant. Is there a way to make this screen delay longer?
The other issue sounds like maybe you have a custom built kernel driver that you need to build for your new kernel before you boot (maybe a SCSI/RAID driver)?
No, using the standard kernel that was in 4.3, and it works.
Gerald