MB-F1 and MB-C2 both specify double flares for CMP trucks. So does the Ford shop manual for Conventional and Modified Conventional vehicles. I don't have access to the corresponding Chevrolet manual. With the use of double flares being so widely specified (up to current vehicles), I would assume your truck probably used them too. If this is correct, then the fittings and junction blocks would all be designed to match the thickness and profile of double flares and might very easily not seal properly on a single flare. My understanding is that double flaring also reduces the chances of the line cracking from the free end. Since single flares are cheaper to make, I think the manufacturers must see a benefit in using double flares that outweighs the added cost.
There are a variety of double flaring tools that vary widely in price and ease of making acceptable double flares. Nickel-copper and steel lines are generally easier to form than stainless steel. Among the cheapest I found on ebay was
http://www.ebay.com/itm/Tubing-Doubl...610665&vxp=mtr
I did all-new lines for a C15A in stainless using this
http://www.eastwood.com/professional...ring-tool.html without any spoiled flares. I had done some stainless flaring with a tool like the cheaper variant with mixed results and decided I would go with the Eastwood tool when doing the complete truck. I found it much less frustrating to use than the simple traditional (cheap) machine. One limitation is that it can only flare up to about 1-1/2" from an existing curve in the line. It may work better to plan your lines to flare and then bend if possible although this gets harder to do if the same applies at both ends of the line.