Just me but I am going to bet that the distance from the back of the bellhousing or back of the 3.8 block to the center of the crossmember is going to be different between the back of the block on a 302W or the back of a 94/95 bellhousing or back of a 85-93 bellhousing. Reason for this is the blocks are different and the bellhousings are different.
What I would do is bolt the block to the bell to the transmission and stab that into the car. Get the shifter in the ideal location and block all of this up safely. From there you now know where your transmission cross member has to install, where the motor mounts need to install and you can verify that the engine with accessories on it will clear the radiator with whatever fan combination you decide to use.
Anyhow that is how I would approach this.
What I would do is bolt the block to the bell to the transmission and stab that into the car. Get the shifter in the ideal location and block all of this up safely. From there you now know where your transmission cross member has to install, where the motor mounts need to install and you can verify that the engine with accessories on it will clear the radiator with whatever fan combination you decide to use.
Anyhow that is how I would approach this.