Never built either, but B/RB speed parts have been around for so much longer so my gut tells me it would be cheaper and easier than the hemi to get 650 hp right now.

Used 5.7 Hemi engines are $1500-$2000 on CL, but they don't make 650 hp either. You would need a power adder, or a larger engine as a starting point.

If you post this in the race-only section you may get some more experienced opinions.