my Low dollar 318 hop-up project

Yup. You did. Then jpar said to take your $220 that you’d save by making your own subframe connectors and go buy a set of cheap summit headers instead of building crap like roadkill. Lol
Headers
1971 PLYMOUTH DUSTER Summit Racing® Headers SUM-G9040
Exhaust straight pipe
Summit Racing® Exhaust Tubing SUM-640025-1
Mufflers
Summit Racing® Fully Welded Turbo Mufflers SUM-633222
Hangers
Pypes Exhaust Hangers HVH10S
All this above Or if your included to be a little spendy and press the easy button... OK, not really spendy, there about equal.
https://www.summitracing.com/parts/sum-680144/overview/year/1971/make/plymouth/model/duster