Hi Jules, You may want to use more than one car battery, but one will do for a start.
1) N-Mos Fets tend to be More Relialable than P-Mos, same goes for NPN and PNP Transistors, NPN are Better with Much Lower Leakage. (Very Simply Put, Mainly due to internal Construction.) And Current Sinking is almost always More Efficient than Current Sourcing.
2) Also, N-Mos Fets have the Tabs as the Anode, therefore in this circuit, all can be placed on a common heatsink and the Anodes are typically connected together via the heat tab.
3) Stall currents: probably at least double. (60 amps? or so) Because of this, and/or the possibility of Mos-Fet Failure, A fuseable link is highly recommended and it can be easily made using a 1 inch long piece of a smaller copper wire, (maybe a 20 AWG) inserted into the Anode lead coming from the battery positive.
4) Seperate ground returns help Equil out small differences in the Anode to Source between each of the mosfets.
5) A direct Return of the Source's to the batterys negative is necessary to prevent voltage drops being created that can affect the 555.
Need more help Jules, just e-mail me direct.
Take care.........Gary