As a purely academic question spdsk8tr (and lease don't take my questioning the wrong way), did you make any provision for offsetting the engine to the passenger side? My reason for asking is that w/the JTR mounts, the drivers' side provides for an offset of ~3/4" to the passenger side of the car.
Viewing the JTR manual, the chevy mount on the driver's side is move 1" toward the bottom of the mount as compared to the datsun mount holes. Due to the angle that the plate is at when bolted to the Chevy Engine, I can see that this would move the engine over a certain amount and maintain it level side to side (or am I just assuming this also?)
Both Z & ZX use the same motor mounts, and by looking at the MSA system, they use the stock datsun mounts and leave the Chevy mount off - simple spacers.
I personally don't think that the datsun mounts were designed for the HP and (esp) torque of the SB, so I definitely think that spacers utilizing the chevy mount is the way to go, however - the offset ? bothers me.
JTR all said and done has a total of 1 9/16" (1.831") for spacer & setback plate thickness w/no offset on the pass. side. The driver's side has a total thickness of 2 9/16" (2.831") with the aforementioned 3/4" offset to the passenger side. The amount the engine is moved down on the drivers side in relation to the motor mount position seems to 'even out' the difference in spacer height, therefor the engine is level side to side.
Assuming your memory of your measurements is accurate (driver's side so close that it doesn't bear mentioning and passenger withing 1/4" between both systems discussed), then there appears to be a distance gap involved (heightwise and between where the chevy mounting bolts line up in relation to the engine mount towers.
With straight spacers the specified (and seemingly correct height) and no offset on the driver's side, it would seem that the engine would be centered in the car, and likely the engine would not be level 'side to side'.
I am simply trying to visualise how just spacers would work without that offset on the driver's side...
Inquiring minds want to know!
I personally wil keep my '83 ZXT turbo'd - the '75 is for a V8 - but I'm gonna do my own crossmember for wicherver engine I choose and not go JTR (if not LS1) because I want as much room in the engine compartment as possible for accessories and turbos. Beside - it will look cleaner!
Once again, I personally do not think much of the MSA kit for the ZX (haven't seen for the Z) because a V* should be using motor mounts designed for IT - not another engine. There is too much engineering that the car companies go through for NVH to toally discount so flagrantly, not to mention sheer stoutness-wise in this case.