RockAuto still has kits listed or do the Holley conversion and there is one ebay already rebuilt
http://www.ebay.com/itm/1969-GMC-car...238a22&vxp=mtr
stromberg 23-228 381270, 381285 carburetor with manual choke
1968 GMC C25/C2500 PICKUP 5.7L 351cid V6 : Fuel/Air : Carburetor Repair Kit Price
GUARANTEED PARTS Part # 96154
Carburetor Kit 2 BBL,STROMBERG EQPD; Engine Code 351 [Wholesaler Closeout -- 30 Day Warranty] (Only 4 Remaining)
$4.26
STANDARD MOTOR PRODUCTS Part # 429 {Click Info Link for Alternate/OEM Part Numbers} Hygrade Jiffy Kit for STROMBERG #23-184
Carburetor Kit 2 BBL,STROMBERG EQPD; Engine Code 351
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$5.62
STANDARD MOTOR PRODUCTS Part # 526A For Stromberg 2 Bbl. Carb.
Carburetor Kit 2 BBL and E ENG CODE,STROMBERG EQPD; Engine Code 351