One very helpful source of ideas for me has been attending historic races and really looking at the cars in the pits, taking a lot of pictures of details like suspension parts, roll cages, assorted brackets, and all manner of pieces. I save all my pictures to a file I named "good ideas" If you see a cool car somewhere take pictures of the details. I get a lot of body work ideas from show car guys who did their own work. They really know body and paint.
Most valuable free catalog, Aircraft Spruce, they have a race car smaller catalog but get the big full one. These guys are building airplanes. A LOT of their stuff will work for us.
Most of my welding information comes from the welder vendors such as Lincoln and Miller. Very good information on their websites.
Send for and save catalogs. Just the pictures are helpful sometimes when you are trying to figure out how to do something. In nearly every case someone has already solved your problem. The trick is to find it one the web.
Good help on engines, Comp Cams tec line, others are Fuel Safe tec line, Eagle cranks, Calif motor sports for gearbox info.
But most of all its right here on GT40s.com. This is the best single source of advice anywhere on building "kit cars". Lastly ask one of the other builders on this forum directly. I have never been turned away when I asked to talk to some one on this site about something technical. Use the PM system and ask for a phone number and just call em. Most of these guys LOVE to talk about this stuff.
You can call me anytime. PM for Ph #.