Tough to get that low without a dropped axle. Mor Drop in Oakland has been doing it the old way for decades, heating and stretching your axle. They will do a 3 1/2" drop in an old Ford axle for $275. Figure shipping two ways on top of that. Probably near the same cost for one like yours. Call them @ 949-279-8158.
I bought the mono leaf spring for my Dad's 48 GMC from a Company in San Dimas CA. I couldn't find the receipt to get the name again but this company sells the same spring. <a href="http://www.classicperform.com/4754chevtrk.html#springs" target="_blank">http://www.classicperform.com/4754chevtrk.html#springs</a>