If your engine will be set back all the way, it would be a good idea to have a driveline so that you KNOW how far to set the engine forward from the pump.
If you are setting the engine forward for C/G balance, then you will use a longer custom driveline.
Source a driveline and get the overall length measurement. Measure from the bellhousing surface of the block to the crank flange (flywheel surface). Add those two numbers together plus a minimum of 1/8” clearance (some like a little more), and that will determine the fore/aft location of your engine. You could also mock everything up and then just push the engine/driveline forward the 1/8” minimum while you build your engine mounts.
The important thing is that you have some clearance between the yoke and the thrust bearing snap ring so that the pump shaft doesn’t push against your crankshaft causing your crankshaft thrust bearing (and complete engine) to fail. FYI, the yoke on the pump shaft can be trimmed a little if you need some extra clearance, but be careful not to shorten the splines too much. I like one shaft diameter (1.375) of spline engagement, although others have successfully run much less.
