It's the cam...
My last cam had a long duration low ls and wasn't worth crap under three grand but after three grand hold on for dear life...
Had the cam reground and a different animal...
As the cam duration goes up you need to crank up the compression or you loose cranking and dynamic compression which is what keeps the lower RPM torque numbers up. With that cam I’m guessing without doing the math it needs another point of compression to really keep any low RPM torque. Cam selection, head flow and compression need to be matched to have an engine that works great across the band.