not sure what you were trying to show in the tape measure pic but the driveshaft for a th400 needs to be about 3/4 to 1" shorter than the 2004r, and because of yoke differences driveshafts get measured from center of ujoints , so for the th400 that needs a shaft about 50.5" measured center of ujoints on a 8.5 rear . depending on rear used and pinion and trans yokes that can vary so its best to measure and order the shaft after the trans is in and you know what yokes you plan to run
for the trans mount you can slot the openings in the 2004r mount to get it to bolt to the th400 tailshaft or get the hr poly setup
youll also want to swap out the shifter and cable for something for a three speed . the 2004r column lockout assembly will work so you keep neutral start and reverse lamp but youll need to add the2004r bracket loops to th400 shifter bracket, from pic yours has the loops on the th400 shft bracket
if the trans has a brake (rmvb) the track will want you to run a trans blanket or sheild, even if they dont bust your balls you should run one anyway
trans cooler lines can be bent to fit (or run -6an) but be aware the inlet and outlets are reversed on the th400 compared to the 2004r