I had a 1973 B200 , it came with a 318 ,Iinstalled a 68 383 in it . Big block motor mounts ,oil pan the dip stick threaded into the bottom driver side of pan, any big block 727 will work,but the van one has a hole mid-center on the upper part of the bell housing , so you can read the timing off the torque converter,you d not have to have a van converter,just mark the one you use ,BB truck headers work fine .I used Herman’s. The dog house got bigger in 1978, when there was a body style change. If you have any questions please let me know,I’ll be happy to jog my memory.