Are you talking rally as in real performance rally, or just brisk cruising on curvy roads? Either way, pay attention to having a wide torque band. That is a somewhat different build approach than what works for drag racing or circle track racing.
In that case, a higher compression ratio will be important to that use to help with the wider torque band, and that points to either of the Magnum's over the stock LA's. Only the first 3.5 years of the 340's came with decent stock compression ratios (actual, not advertised). The Magnum's come out the box with almost a point higher static CR than their LA counterparts. The LA's can certainly be built to higher compression ratios, but if you are going stock-out-of-the-box, then the Magnums are better in that regard.
Do you have your heart set on buying something and dropping it in, or building something or having it built?