32" tires will require a very high gear ratio.
ratio check Do it again but leave one wheel on the ground. This will result in the internal differential spinning
2:1 differential ( right /left wheel) This will half or double the ring pinion ratio.
You are getting a weird count cause the diff is spinning.
5" manifold vacuum at idle. either its got quite the cam in it or the timing is very retarded.
Find true TDC on the balancer timing tab, using a piston stop.
cammed motor will respond to a different timing curve with much more inital at idle (recurve the distributor)
this will improve the idle, manfifold vacuum at idle and the engine torque.
a low engine compression ratio will not work with a big cam, especially in a 283.