That's a really good combination (saginaw 4 speed and 3.07 rear end). I think the 2.85 or 3.11 first gear saginaw box would be optimal because the gear spread isn't as wide and acceleration would be slightly better.
The 3.07 is too high for a 250 with a 3 speed. The drop in rpms between upshifts would make your truck useless for towing, hauling and acceleration would suffer greatly.
I have almost the exact same setup as you, except my engine is a 230. I bought a 3.07 rear end and a saginaw 4 speed to replace my 3.73/3-speed. Then I came across a GM 4 speed transmission with 0.73 overdrive. I liked the idea of making one swap instead of two. And I'd actually end up with a lower final drive ratio (2.72). So I sold the 3.07 (still have the saginaw).
While I was at it I decided to build a 292 to go behind the new tranny. Might as well do it all at once. The engine is almost finished. It's just a mild cam and flywheel away from being complete. 292 bored .060 over, 500 cfm 4 barrel, headers, HEI. Should be a great performer when finished.
-ftyler
