Here's a circuit I used to get the squarebody ('75) front side markers to function as running lights and turn signals when converting to LED's.
This approach intentionally dims the side marker lights to provide high contrast when the turn signal is applied...it surprising how dim you can go with...
Some notes I had from when I changed my exterior lights to LED...
If the bulb is behind a colored lens (red, for example) use red colored LEDS...dont get too hung up on lumens or wattage power output. The colored lens is a light filter and will only allow the respective wavelength to pass...
What wp29p4a said.
Thomas & Betts STA-KON model WT111M has been my go-to crimp tool for 40+ years.
Uninsulated butt connectors and ring terminals (don't cheap-out, buy good quality brand, brazed-seam if you can find them) followed by waterproof heat shrink tubing (has heat-activated glue/sealant...
as Radiohead points out, high resistance at a plug terminal will create heat and cause subsequent heat damage to switch internals...
My first thought was headlight bulbs have been replaced w/ higher output, higher current devices...this may increase current draw in excess of the rated current...
If all else fails, try googling "hat channel". Its an industrial building product.
Might get lucky if what you need is a standard hat channel dimension.
16ga. mild steel would probably work nicely.
Hat channel + square tube stack a possibility too.
Was looking thru AMD's catalog and noticed they offer the rear cross sill;
https://www.autometaldirect.com/body-components-cross-sills-rear-tail-pan-cross-rail-assembly-complete-73-86-chevy-gmc-ck-fleetside-pickup-87-91-chevy-gmc-rv-fleetside-pickup-w-steel-bed-floor-p-1343.html
Near as I can...