New Castle is located about 6 miles north of Forrest City on hwy 284. It is about 3 miles east of Colt. If you use a map of Arkansas from today and find interstate 40 and exit of mile marker 240 the name of the exit is Crowley Ridge hwy 284 and go north about 6 to 7 miles you will be in New Castle.
I do not know of an Allen Farm but my mother is going to ask around and see if she can come up with something.
I couple of hints for New Castle research. My great grandmother was born in New Castle (Pearl Pickens) in doing research I have found the New Castle is very close to the county line and I have found records in both St. Francis County and Cross County. Also New Castle is on the St. Francis County River in 1890 to 1920 alot of the people who lived in this area were Farmer, Fisherman, and Shell Diggers.
If you have some family names I will give the books I have at home a look for you.