Junkyard 10-bolt now or save for a 9-inch?

If it's an 8.5/8.6, a junkyard 10-bolt can be a real option — it's the one high-performance 10-bolt. The 7.5 and 8.2 are much more limited:

GM 10-bolt	Stock capacity (Speedway)	Notes
8.5 / 8.6	The only high-performance 10-bolt; can hold about 1,000 hp built properly	Large square lugs at 5 and 7 o'clock; flat webs on the front; 28- or 30-spline
8.2 (1964–72)	About 400 hp on street tires, 350 on drag radials, 300 on slicks	Built for racing: about 500 hp on street tires, 400–450 on slicks
7.5 / 7.625	About 350 hp on street tires	Maxes out around 500 hp with maximum effort and expense; 28-spline only, no 30-spline upgrade

When to save for a 9-inch: Speedway notes most Ford 9-inch units handle 600 hp or more, and reinforced ones over 1,000. Direct bolt-in 9-inch housings exist for several GM applications, including muscle cars and C/K pickups. At 600 hp and up, especially with boost and slicks, Speedway advises putting the money into a stronger rear rather than building a 10-bolt.

Telling them apart: measure from the lower center cover bolt to its neighbors — about 3-1/4 in on a 7.5, 3-3/4 in on an 8.5/8.6. The 8.5 has large square lugs at 5 and 7 o'clock; the 7.5's are small. Confirm with the codes stamped on the axle tube.