It's not FE collectible, but a great small block upgrade. I usually see them pop up on CL around here for $100-$200 for rebuildable cores. A roller block will fetch about $300-$400 when you can find them.
Only reason I didn't go 351 is that I wanted a T-5 trans and I was afraid that I would put too much torque to it with a built up 351.
Now for my '95, I keep dreaming of swapping out the 5.0 for a 351 one day. But that requires a lot of pre-planning and expense for an SN95 swap.
1966 GT Fastback, restoration almost there! Code T Red with White LeMans stripes.
2015 GT Fastback - Deep Impact Blue