The 4.3 V6 weighs 425# and the Duramax 835# so you have 400#'s more weight but since it is a truck the front suspension is designed to handle load weight in the bed. If the IFS is from a single cab long bed S10 the max load in bed was probably around 1500#'s which added at least 500#'s onto the front end when bed loaded which is in the design range if no load at all in the bed. Although it may handle the extra weight it will drive like a pig since the front to rear weight ratio will be at least 75/25---helpless on any road with less than ideal traction. My 76 LUV with a 400 sbc was completely helpless if it rained.

Evan