Save yourself the hassle - the bolt and barrel extension are different on the .223 model.
Start with a .243/.308/.30-06, hopefully they are cheap down there like they are around these parts - and add the 7615 magazine adapter.
A portion of the magazine adapter sticks up in the front and will need to be cut down a bit for case body/shoulder clearance.
Remington Parts
Remington Parts Pricing List
Next to the last page of the pricing list is where you'll find the 7615 listings.
I bought the whole magazine adapter as an assembly(armorer contact), but I don't see a listing for the whole thing there.
Here's the related pieces I'd be looking for.
F300969 V-I 7615 MAGAZINE INSERT SCREW $ 3 .20
F301032 V-A 7615 MAGAZINE INSERT $ 3 9.00
F300964 V-C 7615 MAGAZINE LATCH ROD $ 4 .50
F300963 V-D 7615 MAGAZINE LATCH BUTTON $ 3 .70
F300965 V-38 7615 MAGAZINE LATCH SPRING $ 3 .20
F300962 V-37 7615 MAGAZINE LATCH $ 1 7.00
F400357 V-43 7615 REC ASSEM 7615 MO BRUSH $ 1 76.00
F91436 V-43b 7615 RECEIVER INSERT $ 4 .10
F14242 V-44 7615 RECEIVER PLUG SCREW BL (.162 x .40) $ 3 .20
F18551 V-45 7615 RECEIVER STUD (7/16-28 UNEF 3A) $ 3 .30
I don't remember buying these parts - but if you could find a good illustration of a 7615 it would help.
F300991 V-F 7615 SHELL STRIPPER $ 3 8.00
F301014 V-H 7615 SHELL STRIPPER GUIDE PLATE $ 6 .80
F301081 not shown 7615 SHELL STRIPPER PLATE SCREW $ 3 .20
F300990 V-E 7615 SHELL STRIPPER GUIDE $ 1 5.00