do you mean the truck has had an ifs cross member installed at one time and there are holes in the frame where it used to be but it is now removed and you have a new one to install?
it doesn't matter how you slice it, you simply need to find your axle center line and go by the instructions to get the new cross member in the right place. if there are holes previously drilled they may need to be welded closed and ground smooth so you start out with a clean slate. trying to make your new cross member fit in the spot where a different cross member has been isn't gonna do anything but make you wonder why you tried that. if you try to redill the holes to match things up between the old and the new you will have made the frame weaker, if they don't quite line up, so the holes are oblong, the bolts will be under more stress to keep things from moving.
got pics of what you have there?
check the frame for straight and sag before you start anything. level side to side and block up all 4 corners. leave room to work. then do some checks.
http://chevy.oldcarmanualproject.com...51ctsm0202.htm
one of these assembly manuals should have a frame drawing with dimensions etc or you could ask on here if someone has the manual and they could send you a pic of the frame dimension page. that should have all the holes etc laid out for from a stock perspective.
http://www.classicindustries.com/pro...ts/tm1047.html