'Are there sites/resources that you found particularly helpful with regard to the overall plumbing of the engine bay and connections through the firewall?'
The publication is AN 01-60JE-4, Airplane Parts Catalogue Army Model P-51D, British Model Mustang IV, 25 July 1944, Revised 25 October 1944, Printed by Wm. B. Burord Printing Co. 10-25-44.
Here, in CD form: https://www.ebay.com/itm/WWII-North-American-P-51-D-K-Mustang-Parts-Catalog-Aircraft-Manual-CD-COPY/264022023371?hash=item3d78f05ccb:g:7r0AAOSwn6Jb3iIj
Found here: http