I spent some time last night measuring the engine block mounting bosses relative to the bellhousing flange on both the 4.3 V6 and the LM7 engine.
The 4.3 has a distance of 9 13/16" from the flange to the rear upper bolt boss for the enging mount.
The LM7 has a distance of 10.27" per some online info (I measured 10 1/4" with my combination square) from the flange to the upper rear bolt boss for the enging mount.
Give these two distances, if I was to put the traditional clam shell mount on the LM7 engine so that the upper rear bolt of the mount is shared by both engines, that would put the new LS based engine BACK by 7/16" from where the 1985 OE 4.3 V6 was mounted.
While searching the internet, there are is not a adapter plate that makes this work with the OE 4.3 V6 frame or motor mounts. Yes, the frame mounts are different for the 43. compared to the SBC of BBC frame mounts. The readily available 3" forward mounts look right, but most are machined for using flat head screws to mount the traditional V8 engine mount.
There is one exception that I have found. The Holley 3" forward mount uses the flat head screw to mount the adapter plate to the LS block on 3 of the bolts and the upper front bolt is shared. The other 2 bolts for the traditional mount are drilled and tapped into the adapter to use a standard hex head bolt. The Holley part number is: 12622HKRUPC. Here is a picture of that adapter plate:
For this one to work for my idea, they need to be mounted opposite of Holley's intention. I.E. the Driver's side mount need to be used on the Passenger side and vice versa. In the picture above, the bottom one would go on the driver's side of my LM7 to make this all work. Best price I found on the net so far is about $80. I will still need to get new rubber mounts also.