Dave, when dealing with a low compression engine, there are a few things to keep in mind about engine modifications. The biggest one is the camshaft. Be careful of the cam duration and also the lobe separation angle. These 2 factors are critical to cylinder pressure. The best way to cam these engines is with a shorter duration, high lift cam. Also look for a camshaft with
110-112 degrees of lobe separation. Lobe separation is often overlooked in cam selection, but it is critical. The higher separation angle will do a few things. Fist, it reduces valve overlap, which is the time that both valves are open during the exhaust stroke. The reduced overlap will improve low RPM torque and idle quality, but at the expense of top end power. Bringing the lobe separation angle down, conversely increases valve overlap which will decrease idle quality somewhat, and also improve top end power.A few other things to keep in mind. LPG likes high compression, so if you do rebuild the engine, take that in to account. Your idea of aftermarket intake and exhaust are also highly recommended, but again be conservative on manifold runner sizes and header tube diameters. The smaller passages will keep the air velocity higher at low engine speeds, which will improve torque, drivability and efficiency. The Edelbrock Performer is a good manifold choice for these types of vehicles. Keep the exhaust header tube diameter to 1.625" and find the headers with the longest primary tubes you can. This will improve low to midrange power.
One thing that might improve your situation very inexpensively is ignition timing. Because of your low compression ratio, you should be able to increase the ignition timing with the fuel you are using. Unfortunately this is probably not just a "turn the distributor to advance the timing" fix, but would probably require lighter springs and different weights in the distributor. I believe Mr. Gasket still offers an AMC advance curve kit which would fill the need pretty good.
I guess the timing would be the first thing I would mess with, because it is the cheapest mod you can make, and next would be the headers and exhaust, based on return of investment to power ratio. Again, just be careful not to get too big on the tube diameters. Just keep in mind when selecting parts, that your goal on a low compression engine is to keep the air velocities up in both of the intake and exhaust passages. Do that and you will be fine.
Chris