Bob knows what he is talking about. When you switch frames you have to worry about the fit of every thing, fit of wheels, engine, tranny, rear, driveshaft and body mounts, etc, ect, etc. There is a lot of extra work. It is much easier and most likely cost effective to buy a mustang 2 crossmember and a crate SBC, if you're looking to modernize. If your frame is in good shape I say keep it.
But in the end it is your call, some people do swaps because they have the parts, time and know how. My truck isn't on the og frame, but I wish is was. Everything would fit better and finding replacement parts would be easier.