[masraum]

In the Chevrolet world, the only 350 is a small block unless some of the new one's are considered big blocks, but I don't believe so. The "big" or "small" refers, I believe, to the outer dimensions of the block, not the cubic inches. I think Pontiac has what is considered a big block 350.

Hydraulic lifters have a small piston inside the lifter with a spring and fluid behind it. If the forces get too large or too fast, then the fluid can bleed out and the spring compress which causes the lifter to essentially shorten meaning you aren't getting the lift and duration that the cam is ground for. Hydraulic lifters are not normally acceptible for use with cams with very aggressive grinds, at least, not back in the day. Hydraulic lifters when run at high rpm with aggressive grinds can cause valve float because the non-rigid nature of the lifter allows the valve to not open/close completely.
Solid lifters do not have springs and hydraulic action, they are rigid and always the same length. They were usually used on the highest performance engines because they could be used at higher rpms and with more aggressive cam grinds.