yes double check the timing
one mistake i see alot when guys change out belts trying to set timing is they get it a tooth off OR they dont bother timing the oil pump sprocket. Yes the oil pump need to be timed also, it will not keep the engine from running but it will cause a harmonic vibration at certain rpm's if not timed correctly.
As for the power loss under load?
-make sure the mixer is clean and it's diaphram is ok
-check and clean out the converter
-make sure the dwell setting is correctly set (via the mixer, this is not an electronic setting like it was with the points system, this is a air/fuel setting on the mixer hidden behind a expansion plug)
-make sure the fuel lockoff is opening completly when energised
-double check vacum lines, make sure they are connected to the mixer correctly to the right ports (yes the vacum advance being connected to the wrong port will keep the timing from advancing correctly when accelerated)

just a few thoughts that come to mind