As
@beans-on-toast said, make sure each compartment has a matching set of speakers. Mixing drivers in a chamber will reduce power handling and put both drivers at risk.
If you get the alnicos reconed and installed, the power handling of the cab drops to about 240W...so you can't max out the amp without risking the alnicos. FYI, Back when Ampeg equipped 810s with alnicos, you needed two cabs to handle the full power of an SVT.
The drivers are 32 ohms and connected in parallel. 32/8=4 ohms. Because the drivers are all 32 ohms and they are connected in parallel they will share the power equally. At 240 the power is split eight ways. 240W/8=30W. AFAIK 30W is the power rating of the alnico drivers.
A lot of people seem to think the Heritage drivers sound very close to the Alnicos. Unfortunately the Heritage driver is considerably more expensive than the regular 810 driver, and I believe it has been out of stock for quite awhile:
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This is the regular OEM driver:
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Highly Regarded Eminence replacement driver:
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You should be able to find reviews compairing the regular OEM driver to the B810. Soundwise, they are close but not identical. Specs say the B810 handles 50W more.
Since the Heritage drivers seem hard to come buy, consider buying four OEM drivers or four B810s. Put those in the bottom of the cab and put the best four vintage Eminence drivers in the top of the cab. Or if you find you like the new drivers better, put the new drivers in the top of the cab. You could even alternate driver type every other compartment if you like...just make sure you have a matching pair drivers in each compartment.
Recone the alnicos and sell them...either that or start hunting down 6 more alnico drivers. Be advised that Ampeg used different brands and models of Alnicos as well, so keep this in mind when you are pairing up drivers.
Do stainless steel strings loose there sound quicker than other material