Are you referring to the use of a Motorcraft (Ford) 2100 2-bbl carb.? This is becoming a fairly common swap for the 258 engine. Depending on your current setup, you may or may not have to use an aftermarket intake manifold. As long as your current carb. is a 2-bbl, you should be able to get away with using a carb adapter (Mr. Gasket part #1937 or Summit part #TRD-2086).
The Motorcraft 2100 series carb. comes in 3 different models which all have different CFM ratings. The model you would want to look for is the carb. that has the number "1.08" in a small circle on the driver's side of the float bowl. This represents the lowest CFM rating of 200 that should work the best for you.
The Motorcraft 2100 series carb. can be found on numerous vehicles. Here is just a small sample of vehicle with the engines you might want to check:
FORD: 302, 330, 350, 360, 351, 390, 400, 429
Hopefully some of the above info answers a couple of your questions.