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Don't forget crumple zones...
Hello jojogunn and cRiPpLe_rOoStEr,
Re-engineering the drivetrain isn't nearly as complicated as re-engineering a crash safety system. Assuming the donor isn't bent out of whack, your idea of using the chassis as a "jig" has merit, though it may be easier to obtain measurements of the relevant mounting positions (motor mounts, transmission/transaxle mounts, suspension arms/strut mounts, steering rack mounts, etc.) and build a replacement frame to suit. In some cases, an auto body repair manual for the donor in question will have all the relevant measurements to assure that your new chassis meets the requirements.
I've been thinking about adapting older "classic" body styles to newer chassis for a while, with an eye to maintaining the more modern crash safety mechanisms. This would apply to using hybrid chassis as well. Note that I've only gotten as far as "frontal impact", not much else. Side impact is a lot more involved, what with appropriate crush thickness of the doors, etc.
What I've been able to conclude so far is that you need to keep everything forward of the firewall from the modern donor, and use the donor's seats, seatbelts with pretensioners, dash with steering wheel, airbags and electronics, and pad the interior of the new passenger compartment at least as well as the donor. Seat to steering wheel alignment is important as well, both vertically and longitudinally. Stiffen up the passenger compartment of the new vehicle to form a 'safety cell', but *don't* extend the stiffening to the front suspension mounts or beyond. If using a body on frame donor, keep as much of the donor's frame as possible, and tie the body to the frame using OEM body mounts/bolts. From crash test photos, sometimes the frame bends further back under the passenger compartment and rips away from the body a bit.
The idea is to maintain the modern crumple zone's energy absorption profile in a crash, and maintain the integrity of the passenger compartment so it does not collapse or bend in half or similar.
Monocoque cars make this harder, especially as welding to a unibody is quite an involved process to get right, especially with modern "mixed steel" body designs. There are specific places in the unibody structure that use high strength steel that requires very careful work to weld, and in some cases the manufacturer and the aftermarket body shops do not consider them repairable at all. Full-frame vehicles are somewhat easier as the sheetmetal doesn't provide a very big percentage of the overall crumple zone's energy absorption.
Keep the overall mass of the vehicle similar to the modern donor as well, or at least stay within the GVWR.
If you are not retaining the airbags and pretensioners from your modern donor, please fit replacement seatbelts that are designed for a vehicle that never had airbags or pretensioners - they stretch different.
Do note, though, that I don't think using a modern chassis with classic sheetmetal will generally end up worse than the classic was in the first place, even with welding on mixed steel unibodies - so long as your major welding is all behind the A-pillar. Most modern cars crumple better up front than the classics, and have collapsible steering columns, etc. so that's all upside. Modern suspensions handle better, too, so you are less likely to get into an accident in the first place.
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