I'm an automotive tech that apprenticed at an alternative fuel conversion shop. I pitched a tent every time I saw a dual quad setup, and when it was time to rework the ol' Dodge, I figured I would throw in my own little twist.
The 'carbs' sit on Holley 4 barrel throttle plates and two were used because they flow 425 cfm each. If you've ever seen the pic of the propane powered '68 or '69 Charger that was campagned back then, it had a near identical setup only differences being it had 600 cfm units and the regulators were mounted vertically.

So far as engine mods go, I used a 68 block punched .030", steel crank cut .010/.010, TRW 6pak replacements, MP 284/484 cam, 906 heads with hardened seats and stainless valves. Only thing I had trouble with was cam timing. I originally set it up 4 degrees advanced and it would barely run. Thought about trying retarded cam timing but opted for straight up. Up here propane is 60 cents/liter where premium is over a buck. Propane has higher octane although it's critical to keep the incoming air cool if you're running higher comp ratios. Phoenix summertime traffic was hard on her, Would detonate like mad if I layed into it after sitting a a traffic light for a couple of minutes.