Welcome to the forum. From the sounds of it you are planning to build the cab into one of the fenderless, so called "bobber" trucks. Is that right? If so, what you need to do is mockup the cab with some blocks under it to get it sitting at the height and attitude you want, then slide your rear axle up behind it, using the tires you will be running. Then place your engine in front of the cab in what looks like a reasonable spot, and finally put your front axle and tires in front of that in a position that allows for radiator clearance and a good appearance.

Now sit back in a chair, open a cold one, and take a long look at the truck to see if you like it. Shift things around to get it perfect, and then you can determine how much kickup you need front and back to get it all put together. There is no other way to properly calculate kickup and frame length IMO.

Don