So a 6bt is only 300lbs heavier than a w123. And I'm taking my time with it don't care how long it takes or how much money

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This tells you everything you need to know about whether this project will get done. Someone who walks into this without a time or cost budget isnt serious. Same as someone who says "the only hard part" about the simplest part of the swap.

The hardest part here will be making something functionally usable after cutting out the first 3 feet of the transmission tunnel, and hacking out quite a bit of structure to get the shifter within a foot of where it could reasonably be used. Hacking a car apart to rest a huge engine in it is one thing, reinstalling the structural stability and indeed beefing it up to hold the weight after the fact is something else.
The video above shows that perfectly. They rip out a bunch of structure to wedge the engine in, and you can see how floppy it is. Though it is indeed an impressive job and those guys have the skills to finish it.

Personally what i would do is start with a 126, with a nice long hood. Then id build a chassis custom for the engine and rear diff combination i wanted that solved all my strength and suspension problems. Then id drop the unibody modified to fit on my chassis. Would probably be the same amount of work. I might even start with a truck chassis and lower and shorten it. Benefit would be you could still tow something. Having a 6bt in a car unibody that is too weak to tow much of anything would be like a woman getting a single breast implant. If you are gonna do it, go all out and get the most out of an awesome drivetrain.

Now hopefully having his chops busted will have the desired effect of having him do it just to spite all the naysayers.

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