Mileage:
1. Try advancing the timing a few degrees. (More BTDC) If your engine 'pings' under load (acceleration) with the extra advance, back off a bit on how much you've advanced it.
This helped my MPG on my '66 Dodge Polara (383), '69 Dodge M300 (318), and '77 Dodge B200 (318). Advancing the timing gives the cylinders more time to burn the fuel charge before the exhaust valves open.
This may also help pass the HC part of emissions testing. (HC = hydrocarbons = unburned fuel in the exhaust system.)
2. Change your driving style, especially accelerating from a stop. Double or triple the distance it takes you to accelerate to speed. (Try thinking like you're driving a heavily loaded 18-wheeler and ease your way up to speed.)
-1 vs. -3 engines:
The main difference between 'standard' (-1) and 'industrial' (-3) engines is the crankshaft and, possibly, the camshaft. Most 'industrial' engines have a stronger forged steel crank instead of the more standard cast steel crank. This is why race engine builders like to cannibalize motorhome engines.
The only sure way to tell if your engine is a -1 or -3 is to drop the oilpan and look at the crank. (Cast cranks have a narrow parting line and forged cranks have a wider parting line.)
The best way to tell is to look for the engine serial number stamped into a machined pad on the engine. (On a LA small block, it's on the front left just below the left cylinder head ... I can't recall the location on the B big blocks.)
Both types of engine used the same blocks so the block casting number is meaningless for telling the difference.
For most of us, discussion about -1 vs. -3 is moot. Broken crankshafts have been extremely rare among everyone except drag racers since the mid-60's and, hopefully, you're not stressing your engine to the point where 'standard' versus 'industrial' becomes an issue.
