The 518/618 units get run behind heavily modified truck that weigh upwards of 8k lbs putting out over 600hp and 1500ftlbs tq. The trans does need to be upgraded of course to take it. There are specialty converters running up to 4 disc lock up clutches in them to put up with this abuse. Of course they are expensive too. Another way to go would be to get a donor trans from a 93 back diesel or early production v8 from like 89. They did not run a lock up converter.