Go with the bushing if that's what you want. As far as a custom converter goes, that is not an issue, even if you want/need some wild stall speed. The converter shop I worked for did many of these converters, with the 18 spline input shaft and the 1.8" pilot. For any competent converter builder, NO PROBLEM either stock or high stall. There are two ways this is done: one use a front cover that has this size pilot, or secondly machine a pilot of this size and weld it to the front cover. Seen it done many, many times.