Here the subordinate conjunction that was accidentally omitted in the 1841 British edition; the subsequent LDS edition (1849) restored the that. Either reading is theoretically possible (see, for instance, the discussion under 2 Nephi 30:2 as well as more generally under that in volume 3). In each case, we follow the earliest textual sources, thus “this much I say that there is …” here in Alma 40:21.
Summary: Maintain in Alma 40:21 the subordinate conjunction that after the verb say: “but this much I say that there is a space between death and the resurrection of the body”.