My doomed 408 build

That's so awesome that you seem to have straightened out your engine problems!

If you go with sub frame connectors, which you SHOULD, I would use the good weld-in ones like these from US Cartool:
Subframe Connectors

They also sell complete body stiffening kits like these, if you wanna get crazy:
Chassis Stiffening Kits

Or there are many write-ups on this forum where guys make their own that tie in through the floors. The bolt-in ones really don't seem to do a whole lot, from what everyone says. I went with the US Cartool connectors. Haven't driven it yet but should be able to in the next week or two with warmer weather.
I sure do like the way the contoured ones from US Cartool look when they are finished.

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