Internet sites are OK for info, but even from site to site can have small dicrepencies in data/measurements and many sites just copy the info from other sites anyway (they are saving money too!).

When I designed ACAD plans for my custom tank and gave it to suppliers to price/build. The better outfits said they would like to go to the museum where the actual vehicle is and make all their measurements, which is the best method.

Getting a 35th scale model for 10 dollars would be the next best research item (from a quality manufacturer) in my opinion. Very easy to scale up from that.