For the price or parts it would take you to do such a thing, I'd just wait for a low-range T-18. In short, it would involve swapping most of the guts from a T-18 into the T-19 case, and of course you see the pointlessness of such an endeavour in terms of saving your hard-earned dollar.
T-19's came in 3 different versions. 4.02, 5.11, and 6.32 low gear. Ford used 4.02 and 5.11 versions apparently mostly behind deisels. IH used both 4.02 and 6.32 versions. If it is free I would take the T-19. Some of the parts will be good in a T-18 and the cases are internally the same. If this is a IH T-19 then even if the gear ratios are wrong then it could have ford T-18 internals in stalled on a HI T-18 main shaft and need no adaptors.