With mobile phone apps and cheap reference microphones it's easier than it's ever been to do this stuff... I would start by measuring up the cab, port and existing speaker and using winisd to calculate the theoretical cab resonant frequency. That will help inform what might be good drivers and then you can model them.
Then I'd make up a rig for measuring impedance curves. You only really need couple of resistors, a phone, an amp and a multimeter. How tos are available on line. If you can do it accurately enough to get actual reliable measurements so much the better, but even a qualitative curve is useful. Then of course the use of a frequency response chart is obvious. The fly in the ointment is that a perfectly flat frequency response may not give a musically desirable response. I recall the surprise I got when I discovered my favourite sounding bass amp had a hpf and was rolling off low frequencies from about 80hz.
Where the experience comes in is knowing what to target for... I made a little ported 1x10 for Simon to match a high end Celestion driver that looked nice on paper, but was horribly boomy in the room. He ended up converting it to sealed. But I used the same design cab with a pair of S/H eminences for a pa rig, and although it didn't look as good to me on paper, when I used them as PA at a little local session I stopped bothering to bring a bass rig and just put my bass in the PA with the vocals. So clearly my idea of what a good bass rig should look like on paper was a mile out!
You know, if you are buying new drivers, there's a lot to be said for 32ohm ones in a 4x10... The other big decision is neo or old school magnets. I treated myself to a neo driver in my lightweight 1x15 bass cab when I got a tax refund and its wonderful. But your Peavey cab is by all accounts rather solid, and they may not make such a difference.
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