Red,
The longgggggggggg wheel base on the m819 is what makes it a far better lift tow wrecker then ANY of the older 5t wreckers (39, 809(except 819) and 939 series), the 819 has All the major weight and most of the minor weight in FRONT of the rear trunion , this allows for a GREAT counter weight, I have NEVER had the front end off the ground even with major lift limits +
The big issue in moving the rears back is the X frame member, this keeps the frame from twisting from sharp turns, the trunion is dead center under the X member which extends about three feet on each side of the trunion, this configuration makes a almost unbendable/unbreakable frame, this strong frame FORCES the rears to follow around the curve instead of trying to go straight. You will notice that most bent MV frames are bent in front of the X frame, no matter they were hit. Most of the newer truck frames DO NOT have the X member because the frame is designed to absorb the impact which in turn is supposed to keep the driver safer.
I would rather have a older military frame then any newer frame, when I wrecked the 819 I hit so hard the the r front spring mount got ripped off the frame and also broke the front gear cover/motor mount, I know for a fact that if it was not for a OLDER military frame, I would be posting this using worm.com.