if you haven't got one, here is a link to the factory assembly manual. you may want to download this for future reference in case the page gets deleted at some point. this will have frame drawings with dimensions for the stock trucks. note that the 58 and 59 have a longer frame out ahead of the front axle because the sheet metal is longer so the rad support mounts are not in the same place as the 55-57 trucks.
https://www.trifive.com/d1/55-59Assy.pdf
like noted previously by others, if you plan on running a stock ride height then the stock axle locations will work. if you plan to run a lower than stock ride height then the front axle usually gets moved ahead an inch or inch and half. this can vary depending on the tire diameter chosen as well as the ride height chosen. really, it is best for you to have a tire of the size you will install when done, even if it is a baldie, so you can mock it up in the wheel opening with the truck sitting at the ride height you want when done. because the front wheel opening is rounded in the front side and tapered on the rear side the tire looks like it is being stuffed into the rear of the wheel opening as the truck is lowered. the same goes for the space ahead of the wheel if the axle is moved too far forward.
I like your plan to build a new frame. I have seen a lot of trucks with the front of the frame cut off and a subframe welded on, then the rear of the frame cut off and a C notch welded in. I think, really, the only part of the frame that is original is the section under the cab. why not just start from scratch then and have a nice long section of rectangular box steel running fore to aft until it is required to be cut and reshaped for suspension? the frames are pretty straight between the front suspension and the rear C notch. build a frame table from scrap steel you can pick up cheap, level it all out and start building your new frame on top, it doesn't even need to be the same width as the stock frame as long as you keep the body mounts where they need to be. I really suggest you do some paperwork planning and drawing if you haven't already. I am sure if you are up for building a frame from scratch then you probably have the plan figured out.
post pics as you go please, it keeps us interested and motivated to work on our projects.
what do you plan to run for suspension?
here is a couple of pics of one I did a few years back. TCI MII front end with 2" drop spindles. the string is located 1" ahead of the stock axle center line. this will give you about 6-7" of clearance at the rocker panel front edge with the lower control arms sitting level with the ground. tire is a 215/75r15 I believe.