Using my 45 year old Dart as a daily driver for the last 6-7 years, driving on cobblestone roads, roundabouts and a myriad of speedbumps every day make the 'marginal' spots in the unibody-setup noticable.
Especially when upgrades have been done in the form of 1" Tbars, 17" shortwall tires, better shocks and more blades in the leafspringpacks.
Add in a heavy A518-transmission, 8-3/4" rearaxle and 2 steel propane tanks in the trunk, tighter Borgeson steering, things get loaded more.
(Common) Cracks (already had) appeared in the front frame rails, originating from the rear K-member bolts.
Tried to weld them up and repair/reinforce it, but more is needed to strengthen that area (or add bracing to releave that weaker-area from the stress).
Subframe connectors helped a bunch. But add some basic sheetmetal rust-issues here and there, and other areas are simply experiencing (some large) hairline cracks due to the extra strain on it.