Many moons ago I found bigger exhaust, extractors and improved air inlet made a heap of difference on my old petrol guzzler Cruiser.
As DRW said, regulator must be above gas bottle valve or the liquid gas will stuff it good and proper. Any exhaust place would do a better deal on an exhaust that way it's usually tailor made for your vehicle, extractors a must, a Steinbauer chip would be the shot but you really must have them and the engine tuned to match especially an old one, needs to be checked out properly, compression etc an off the shelf plug an play unit is not the best thing for an engine thats long in the tooth. You may find a run on the dyno at a diesel specialist will show up a worn pump, lazy injector etc which a chip won't improve on.