johnny,
get a 308 block ASAP. You'll have the same economy, and enjoy it more. (the 253 has 3.625" bores, with a head designed for a 4" bore...it makes the gas flow terrible, and every 253 I've owned used the same fuel as a 308).
Try and dig up the works of John Bennet (used to post articles in one of the Classic Car mags, and had some really good info).
I had a WB ute with a 253 on gas. I drilled the EGR so that I had EGR on full time. Restricted one exhaust to keep the crossover heating the intake charge, and changed the vacuum advance so that it was full on at idle (constant vacuum, not ported). Had a copper coil wrapped around an exhaust manifold (wrapped in ceramic fibre blanket), and had the EGR run through that. Put the thermostat in the bottom hose (controlling coolant inlet temperature rather than outlet).
Worked really well...on gas.
Tried to rig a hot air induction (by modifying a Holden V-8 emission controlled air cleaner housing around the remote air cleaner/gas carb), and was partially successful on that...could lean the idle way back, and at WOT, the heating was nil.
Anyway, it was good fun, and I cut my LPG consumption way back.
Problem was that it wasn't that good on petrol, which I didn't want to run anyway.
When running on petrol, I'd add 104+ octane booster to the best fuel I could get. I was only running petrol for 1 tank (70 litres) in 4 tanks of gas (130 litre each), and I was mixing the petrol useage to a day a fortnight to keep the petrol through the system, and not going bad.
I added an upper cylinder lube dispenser (flashlube, but used Lucas in it) to the point that the heated EGR entered under the quadrajet to keep the valves intact.