383 rebuild - kinda whimpy on the dyno?

Sounds about right to me, given the combination. Maybe it's just me, but I remember most stock 383s magnums came with around 10.25:1 compression. Those larger valves don't do diddly in your 906s without some port and bowl work, at the least they need to be modded out to the D.C./M.P. porting template set. And your probably getting close to the limit of what those stamped stock rockers are going to be able to stand before the dreaded "push rod push through" happens. First step, do a compression test and see if it matches the cranking pressure on the Dynamic Compression Calculator. Wallace Racing: Dynamic Compression Ratio Calculator