Doug,
OK, point taken. However, the proper use of a torque wrench will allow for the equal application of clamping force on all the intake bolts. If done in small increments, any warping of the manifold will be non-existent or minimal and allow the manifold and heads to seal optimally (isn't that what we are looking for?). I don't think you can apply a consistent 15-20 ft-lb with a 6 inch box wrench unless you have a calibrated wrist. Looking at my copy of "Road Racing the Ford 289 High Performance Engine", "Special Components & Specifications, Group 2 Sedan & GT 40", the recommended torque is 12-15 ft-lb (read aluminum intake here). So, that's a bit lower than your 15-20. The cast iron manifold recommended torque is something like 23-25 ft-lb. If the torque wrench is set properly and used properly, the installer risks very little. Of course if you whale away on the intake bolts, you're going to break something for sure, especially at the corners which are essentially unsupported.
People disregarding the differences in material strength is why we see lots of helicoils in aluminum parts. They treat those parts as if they were cast iron, crank on the fasteners "just to make sure they are tight" and end up stripping out the threads. The lesson here is be careful and know what you are doing before you do it, get help if you need it.