Do bear in mind that 8x10 portraits and 4x5 are different, in that a life size head shot that fills the frame on 8x10 will be close to 1:1 in magnification. Not so in 4x5...

If your 300mm lens covers 8x10, then you might want to see how that might look on 8x10, provided of course you remain at the correct position / distance from the sitter.