Stock and Modified Frames
The kind of Ford truck you're building will determine the kind of frame you should have. The stock frame with stock suspension is meant to flex a little. You should not make it rigid by adding more support or boxing the frame with frame boxing plates. As well, you should not overpower the stock frame because it just can't take high horsepower engines. I've seen a lot of frames that were just butchered to fit a big block engine in. My first truck had the cross member that connects the front cab supports stripped out to accommodate a 390 with a C6 transmission. That led to the front cab mount wings sagging and cracking. All of the stock cross members are there for a reason, so don't modify them if you're not sure of what you're doing.
If you are adding a Mustang II, Volare or Custom Independent front suspension to your Straight Axle Ford F-SERIES, you should box the frame rails and add as much support as you need to accommodate the engine power. These suspension options do not require the frame to flex. Its better rigid.
If you're running Mickey Thompson's out back, you'll be cutting the frame to accommodate the wide tires. This requires some serious cutting and welding technique, so make sure you measure well and cut and weld in enough support. You'll probably be adding a 4-Link Rear Suspension out back to help get that power to the ground.