I put a camaro t5 in my 59 recently. Had to change the clutch disc because of the spline count. The input shaft was the same length as the original trans so nothing was changed. The drive shaft got shortened 3 inches and the tail housing and top cover were changed to an s10. As for the spacer plate, If the trans you found has the chevy mount pattern you just bolt it directly to your bell housing. That trans may have the ford mount pattern.

Tommy
59 apache 1/2t
261 short stepside