Hi Tabitha,
You will find a great resource in reviewing the tips and techniques explained and illustrated in David Duckett's Night Prowler B-29 build:
viewtopic.php?f=2&t=2352I too just recently did my first Guillow's build with the infill technique, and David helped explain several key things to do when attempting it.
Especially review pages 2 - 6 of his build log, and if you still have any questions, there are plenty of people here willing to lend a helping hand.
Best of luck with the infilled Mustang.
Pauli72