I don't know how wide a cab off a "51 GMC big bolt truck" is but, find a chassis with steering still in tact that had a V8 in it. Chevy, GMC, Dodge, Ford, whatever. Plenty of options, but you have to start measuring various platforms for width and what I said above. Isn't it an option to find a chassis from that era and update it? Borgeson makes steering components to do what you want (go around an engine) and they have the design parameters on their web site. As has been pointed out, you will not be able to just imagine how to do it. You have to find the parts by careful measuring and then still have to mock it up and then design the steering and all the other stuff after you have the basics. Unless you find someone who has done exactly what you want to do. Steering design is not your biggest concern. I'm just repeating what Grigg/46GMC are trying to tell you. Now you have two engineers and a machinist with experience building various trucks chiming in.

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