mono jets were from later trucks. The original carb would have been a Rochester B model. I believe that you might still be able to purchase a rebuilt "B" from NAPA to this day.
http://www.napaonline.com/Search/Detail.aspx?R=HOL647073_0180191135#
Looks like about 200 bucks
You might find them on eBay for less.
Many people dislike the B due to warpage and thus difficulty in them remaining leak free. Rebuilding your monojet may just be a better way to go. Or you if you intend on adding a split manifold and other mods, maybe a 2 barrel carb might be something to consider. Something like a Rochester 2GC. You will need a different intake or use an adapter to your original intake.
2GC carb (good price and upgraded)
http://cgi.ebay.com/ebaymotors/Roch...ck_Parts_Accessories?hash=item4cecd00710
2 bolt to 4 bolt adapter
http://www.parts123.com/parts123/yb.dll?parta~partsort
Offy 5710 from Charlie Price's Vintage Speed site

Last edited by k10; 12/29/2009 3:34 AM.