Sorry for the delay, been too busy elsewhere. The newer housing I had that wouldn't fit had the shaft bearing snap ring cover on the passenger side of the housing facing the frame rail. The older ones that did fit have the snap ring cover on the bottom just in front of the mount pad. They also have the two "ears" for mounting a floor shifter shaft bracket which were absent from the newer type housing.
I went out to the shop and measured the empty transmission I use for mocking up. From the forward face of the tailshaft housing to the center line of the mount holes is 6.75".
Has the transmission cross member been replaced? If so is it too far back? Are you using the early square block trans mount? See image, or has the later model round mount been fabbed in?
The fore/aft engine location is set by the location of the transmission mount. If the engine is too close to the firewall it is the transmission mount and/or crossmember that is causing it to be there.
Checking crossmember location is done as follows. Study the attached measured drawing. Raise car level above a clean floor. Suspend a plumb bob from point "B" on the K-member, either theleft side torsion bar pivot hole or right side, it doesn't doesn't matter. Mark the floor where the plumb bob comes to rest. Next locate the gauge hole "C" or "D" in the crossmember diagonally opposite from the point "B" you have chosen. Suspend the plumb bob from the center of the hole and mark the floor where it comes to rest. Now measure the distance between the two marks on the floor. This measurement should be 46.51 inches on your car plus or minus 1/4".
I'll try to be more prompt from now on, and I think next week I will be able to get under mine so as to get some other measurements that might be helpful. Good luck!