Most Human Body Model (HBM) validation studies have focused on either forward-facing frontal or side impact load cases. In this study, a Finite Element (FE) model of a test setup previously used for rearfacing Post Mortem Human Subjects (PMHS) in high-speed frontal impact tests was developed and used for validation of the SAFER HBM v10 and v11 in both upright and reclined positions. First, tests performed with THOR-50M ATD in the same test setup as used for the PMHS tests were simulated to enable development and verification of the test setup FE model without tuning it with respect to the HBM results, strengthening the confidence in the subsequent HBM validation. Both evaluated SAFER HBM models compared reasonably with respect to the PMHS test data, with average ISO Scores above 0.74 for the X accelerations and displacements. The SAFER HBM v11 showed improved biofidelity with respect to Z-displacements than v10, something which has been highlighted in previous work as challenging to capture both for the THOR-50M ATD in physical testing and the GHBMC M50-O in simulations in the upright position. The potential of HBMs for evaluation of novel seating positions was demonstrated for rear-facing occupants in high-speed frontal impacts and in reclined seating positions.