So let's say you make your motor a stressed member of your frame, like you're talking about. Which part of your 'frame' is the weakest point now? Can you say for sure?
Putting aside the the fact that the motor mounts are not designed for anything but downward and slight lateral loads(slight in terms of stresses your frame routinely sees), there are a few reasons the motor couldn't take it either. For an engine to be a stressed member of the frame, the whole block is beefed up. That engine has three times as much extra metal running between each mount than would be necessary on a motor designed to 'sit' and resist it's own weight in an impact.
For example, imagine a cross section between any two mounts of a stressed member engine, and you'll see big chords of cast metal running through the block from mount to mount. All of that engine's cavities, channels, etc, are designed around those. Stressed member engines are made in that order - "Frame-ness" first; "Suck, Squeeze, Bang, Blow" second. Think about that - a 'stressed member engine' is divergent enough from your everyday automobile engine that the 'working' parts must be redesigned to allow it to perform like a frame when it's subjected to a worst-case-scenario impact. We're talking like an order of magnitude difference in the scale of force applied. And perhaps more importantly, perhaps 10 different kinds of force. Kinds the engine 'structure' was never meant to experience.
Your hacked solution may drive just fine and run for 20 years, I'll give you that. But down the road, when you get into your first 5mph fender bender from an odd angle and your frame contorts (extra, since you didn't bother with a crossmember) and tugs on those engine mounts, the shock will find the weakest, most brittle part of your engine-frame hybrid. That part is the wall of the block, next to the mount, and now you're getting oil all over the road.
Even if you disagree, and decide the motor can take it, the mounts certainly can't. And by the time you're redesigning and beefing up your mounts to deal with the frame deformation that you're trying to avoid, you might as well just put a bloody cross member in and save yourself the potential for finding out the hard way that these little isuzu blocks won't handle frame-scale stresses.
_________________ 75 Mikado G180/4sp Desert Tortoise
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