For invulnerability, edit AITD3.EXE, search for 
26 29 07 E9 3F 02 C4 1E
change to  
90 90 90 E9 3F 02 C4 1E

Play the game as usual and create a save game in slot 1, 
quit to DOS and type the following at the DOS prompt :-
DEBUG
NSAVE0.ITD
L
EAE64 00 40       For maximum health
EB0CE 00 40       For the 38 Special
EB0C6 00 40       For the Winchester
EB1C0 00 40       For the Gatling Gun
EB16A 00 40       For the Shot Gun
EAE7A 00 40       Health after returning from the dead
EB1EA 00 40       For the Colt
W
Q