I know of several frame builders that just clamp a raw frame in a brass faced vice, grab the fork ends, and spread, then measure. You don't need any fancy tools to do the spreading, but you should have an accurate way to check the dropouts to make sure they're centered. The string trick isn't very accurate, IMO, or at least I haven't had good success with it, can't get better than 2-3mm accurate myself. A Park Frame Alignment Gauge is what I use. For perfectionists, a flat table is the gold standard.

The point is you don't need a special tool to do the spreading. As long as you have a way to check for center and measure the dropout distance, you're good. The hub you plan on using is perfectly good if you don't have a set of calipers. Start with just a little bit of force, and check center and distance. You'll have to spread on both sides, typically. If you have indents for chainrings on the drive side, this bends a LOT easier, so keep that in mind.

DIY dropout center tools

DIY frame alignment gauge

The first time I did it was with St. Sheldon's method. I used a DIY frame alignment gauge, but I did have a set of dropout alignment tools.

I also have one of those fancy frame spreaders. Used it once, wasn't impressed. It was easier to do by hand with the BB clamped in a vice. If anyone wants to purchase it from me, PM.

BTW, put a piece of copper, brass, or aluminum between the BB shell faces and the vice, or you'll ding up the face of the BB. With most sealed bearing BB's, it won't make a functional difference, but it scratches up the face and doesn't look very nice. With a vintage, non-sealed BB, there's a good chance the cups won't be parallel anymore.