I'm not very worried about the seat tube length. When you can get a 430mm seatpost with 8" of adjustment range, lots of seat tube lengths are usable. I am a little concerned about the top tube length and height. If the top tube is shorter and the end of the steerer tube is lower, your choice of stems will be limited to the steep, long ones if anything will work. You will need to match the horizontal distance between the center of the captain seatpost and the center of the handlebars, and match the height of the handlebars above the bottom bracket, best measured by getting the heights of both off the ground plane and subtracting. Same thing with the seat tube: match the distances from the center of the bottom bracket spindle, parallel to the seat tube, to the top of the saddle.
One other thing: relative to the length of the wheelbase, you can't shift your weight fore/aft on a tandem the way you can on a single bike. Putting a big captain on an undersize frame moves the center of gravity forward, which loads the front tire more, something that may matter when cornering at the limit. This is a rather minimal concern.