These house rules were originally written by Andrew Martin. Since his website is no longer available, I've mirrored them here.
To make a Tank Destroyer (TD) variant of a Main Battle Tank (MBT), like the German method in WWII, do the following:
All other statistics, including points cost and main gun range, stay the same! This system does violate the DSII points systems somewhat, but we can assume that the removal of the turret compensates for the cost of the heavier main gun.
If the main gun size reaches size six, let that represent a next generation size five weapon, just coming into production. Just draw six chits for damage!
The signature improvement is because of the elimination of the turret and general lowering of the chassis.
Hull down for a Tank Destroyer and other vehicles with only hull mounted weapons is effectively turret down. Use the hull or turret down modifier that makes the most sense in your gaming and is consistent for both sides.
As an option, increase the hull down and turret down die types by one, to reflect the reduced vehicle height. Hull down then reflects the chassis being below terrain with the hull mounted main gun still being able to fire. Turret down means the vehicle hull is behind cover, with the hull mounted main gun being unable to fire.