The only way to be sure of your compression ratio is to measure everything, including combustion chamber volume on each head and how high the pistons go in the block at top-dead-centre (determines "squish") and do the calculations. For LPG you want higher compression to get the best power and economy. Also a fairly high-lift cam with not too much overlap, and perhaps a more aggressive advance curve on the distributor (starting sooner, same final advance.)

With these choices you will need high-octane gasoline, but using as little as you do, the cost won't be a problem. You will save it back with better results on LPG.

Good luck! LPG (propane in Canada) is a great fuel. Engine and oil stay very clean, pollutes less, very good response and power with the right engine configuration. I have heard of people running over 12-1 compression with LPG with great success. Of course, the engine would be very unhappy on gasoline! Around 10-1 is a good compromise.
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67 144, 86 740T, BMW R69S, R60/2