I think you'll find gears are your most cost effective upgrade. Swapping from say a 4.10 to a 4.88 gear will give you an honest 19% increase in torque at the rear wheels and that is at all RPMs from idle up to redline. You would be hard pressed to find any bolt on engine mod that can make that claim.

Not familiar with that engine, not sure what all it was used in or what markets it was used in. If it was used in cars, might be worth checking out some of those forums.