Just to clarify, the pumps are mounted inline but both feed the regulator with separate lines, not the pump in front.  I could should have made that more clear in my first response.

John - I afraid I can't be much help on the cowling as I have not installed mine yet. I don't think an extension is going to be required if you are using the original style cowling (like me) as none of completed 601/650's I've seen have required it. An extension may be required with the new cowling design, but you should probably talk to Jan about that. I plan on using Skybolts or Dzus  fasteners to attach the cowling also. On the fuel pumps, I would think they would really have to be well insulated from the heat as there is a lot of heated air under the cowl from the engine and leaving the radiator. I would also think the pump that was not being used would soak up a lot of heat with no fuel flowing through it. Firewall would also be the worst place to have an unplanned fuel leak (not that there are any good places.....).