[19Fordy]

You can leave frame horizontal as it normally sits. I apply the OSPHO using a spray bottle spraying it into all the tight crevices, joints and double frame rails. I also apply it to the entire external frame using a rag.
Just follow the directions on the OSPHO bottle. Of course, if you can flip the frame on its side or upside down it's easier, especially if you have a rotisserie.
After applying the OSPHO you don't have to neutralize it prior to painting but, it does convert the rust to iron phosphate which may cause 'runs". Let the OSPHO'd metal dry for a day or two before painting. Just smooth the iron phosphate runs out with a piece of sandpaper or small wire wheel. Before you paint the frame I would use a Scotch Brite pad and rub down the frame as much as possible. Do a small area first as a test site. Here's a 51 Merc frame that I did.

There's also a product called Metal Prep. Google it for info.
You might want to post a question on Fordbarn to find out what other folks do to prepare and paint their frame. 50 years ago when I did my 40 frame (below), I just sand blasted it and then sprayed on Rustoleum black. I didn't know about OSPHO. Eastwood makes a nice Chassis Black paint. I am an amateur so there are others who may have better ideas.