It looks like the 6 and the V8 used the same clutch in 1958 trucks, so you will probably be ok there. If your flywheel is worn, you may have to get it resurfaced, because the disc diameter may be just different enough to cause a problem. Set the disc on the flywheel and take a look when you get a chance, if the disc is slightly larger than the one your motor used, the flywheel can be worn enough that it won't seat quite right, if your disc is the exact same size, or slightly smaller, you will be ok. If you have already had the flywheel resurfaced, ignore all of that but do make sure that your bolts will go in far enough that any shoulder on the bolts will not keep you from getting the pressure plate all of the way tightened. Sometimes it takes a pretty good cut to smooth a worn flywheel out, and then the recesses for the pressure plate bolts are no longer deep enough. I suppose washers on the bolts would take care of that, or you could carefully drill the bolt holes for the needed recess.