I am setting at "M" or a little beyond as max advance, tried several positions under the light. Have tried setting the idle advance at "P" and "F".
Michael, First, I am sorry you are having this problem and as you know we will continue to work with you to get it figured out.
This is not typical - the distributor should feel much livelier than what you are seeing. We have used it in many engines with similar specs. to yours with good results.
A couple of thoughts/suggestions:
I suspect you don't have things advanced enough, here's why: As per the instructions, you need to use a timing light with adjustable advance and set for 36 degrees max, this should give you an initial setting of about 10 degrees BTDC. You really must set for max advance and not worry about where the initial advance falls, I think this may be where things are going wrong.
For comparison, the factory "F" mark (1974, US Spec.) is at 5/7 degrees ATDC
and the max advance is 27/33 degrees. That is a big difference from where the ID405 likes to be.
So, if you are anywhere near the factory initial advance mark "F" at idle, the ID405 will very feel sluggish, since you are starting from a very retarded initial mark. This would explain your lack of low-rpm response.
The other wild variable here is the MSD box. Now, I really can't think of any reason why the ID405 would not be compatible with this unit, but we don't recommend it and have not tested it. Our usual set up is simply the ID405 and a Bosch Blue Coil, which works well.