You don't say what type of transmission you are using but it sounds like it must be an automatic since you plan to use the aftermarket front and rear mounts. You will have to mock up the mount locations as GM autos have a wide range of rear mount configurations. If it was a 6 cyl truck you have plenty of room to work with. Set the front mount in place to provide adequate spacing for the distributor and hang the engine to get your spacing right. You can then measure from the rear of the engine where the bellhousing mounts up to get the spacing for the trans rear mount.
Fred
