Hi Cameron,

If your running dual fuel, don't. Decide on gas or petrol and stick with one. Never seen a really good dual fuel carby set up, too much compromise.

I'm a big fan of the std 4 barrel manifold / rochester, HDT would get these really singing and the rochester works really well on the 253 even though on paper its over carb'd.

If your running straight gas use an eldelbrock performer / holley 600 throttle plate / impco 425. HEI ignition is important, LPG heads around 12:1 compression and set initial advance around 12 degrees but get the advance re-curved to around 28 degrees total. Heaps of guys doing this. You'll have no hood clearance problems, cold start is OK - usually around 3-5 secs to prime and smooth idle with no buck and jerk. You might have to go one heat range colder on plugs with slightly smaller gap. Keep it well tuned, run the valve saver on the drip and you'll get better economy on gas than petrol and your motor will last way beyond what is normal with petrol. With this set up on a 308 manual van i've got a best of 14.1 L/100. I had a similar set up on a HQSS that my daughter was driving around and she used to get 11's and this was on the original motor and heads that had done over 200,000 miles and was pretty tired. If you're playing around with the cam be careful because the impco need a nice steady vacuum at idle. Can't begin to tell you how much fun this set up is on my van. I drive the wheels of it and 3 tanks fit under the commercials. I get around 1,000 K's for $110 - $150 (price has been pretty volatile in the last few months)