I can't give you a website or picture because I've never seen anyone else besides myself use this setup. I came up with it because I didn't want to use a chain to limit engine movement. The car was junked long ago so I can't take a picture of it.
http://www.doityourself.com/invt/u606137 , This is the style turnbuckle I used, the one pictured is 1/4 ", I used a 3/8 " X 6" long. I used a studded head bolt in the center of the head, the same ones that the factory uses to attach the battery cable to. I put a flat washer and a lock nut on the stud to secure one end to. The other end I welded a 3/8 " bolt to the frame rail and dropped the other end over the bolt and used same washer and lock nut. The turnbuckle is at a 45 angle from the engine and goes straight over to the frame rail. Just adjust the tension and your done. I imagine you could also attach it to the front of the head surface if you don't want to change the head bolt. In a 2nd Gen firebird you might have to use a different anchor location on the engine as the engine in a 69 GP sits quite far back in the chassis. You might have to attach it to the front of the head or the left front head bolt. Hopefully I have described it well enough that you can fabricate it if you so decide to. I used this setup to supplement the weak design motor mounts that are used in the 69 GP chassis. They broke frequently in my dirt track race car and I started using the turnbuckle and raced the car for 3 years with zero motor mount problems after fabricating this setup. I would use this setup again if I ever have a car that I don't want solid mounts and has a need to limit the engine movement. It worked well for me and I think it has a better appearance than cables or chains, plus it's easily adjustable, and I thought I would pass the information along. Also the turnbuckle is fairly inexpensive even buying 2 locknuts and flatwashers and a good quality 3/8" bolt to weld to the frame, you'd probably have change left from a ten dollar bill. I'd be glad to answer any questions if I haven't given you enough info here.
Brad