A Sheet Feeding System (10) having a reciprocating vacuum shuttle plate (52) includes a knife gate (112) and high-friction rollers 118 and 120 position between the knife gate and an additional conveyor formed by a driven feed roller 94 and an idler feed roller 96. The high-friction rollers form a singulator gap (188) with shoulders 184 of a vacuum-groove (186) on a sheet-engaging surface of the reciprocating shuttle plate. The sheet feeding system includes a transition tray (14) downstream of the additional conveyor for providing a transition to clamps of an endless chain (178). The transition tray comprises an endless conveyor belt (156) with a floating feed roller (160) biased thereagainst. The position of the floating feed roller can be adjusted so as to space it approximately a sheet length from a stationary clamp (176) on the endless chain. The shuttle plate comprises upper panels (76-90), one of which is a vacuum-groove panel (81) which can be interchanged with one another on a shuttle plate frame (74) so that the position of the vacuum-groove panel can be changed. A thumb mechanism 29 provides resistance to falling sheets so that a forward-most sheet (56) has reduced pressure on it.