I am replacing an output Transformer in a 5f1 circuit. I have a larger transformer with 4,8, and 16 ohm output. input resistance is close. This would allow me a greater speaker selection, but I am not sure if there are any other considerations I should know about.

You could try this post on the vintage and modern amp section of the forum. I'm sure there will be a better response of information. Tubedepot also has some good information on the 5f1 and you could email Rob Hull at the tubedepot.com to get more specifics for the transformer you plan to change.