Holley and Edelbrock make EFI systems - 2 grand or so.
If you application is not too far off a stock one, you could probably strip a donor vehicle of everything you need (sensors, computer, wiring, etc)

search the net for 'megasquirt' EFI. A home brew set up that gives you TBI for under $1000, way under if you can get used parts.

will efi give you better mileage? yes, particularly if yo make short cold weather trips.

How much will you drive it? Does it make economic sense to spend the bucks on TBI?

Assume you get a carb for $300, and get 14 mpg. Assume you spend 2000 on EFI and can get 17 mpg. If you drive 5,000 miles a year at $2.20 a gallon the higher mileage will save you $137 in gas. At this rate, it will take 12 years to break even. Longer if gas ghets cheaper (it has started dropping here, down 5 cents in a week).

TBI is cool, cleaner, etc. An OD trans or 2.73 gears would give better mileage for the buck, but so would a 4.3 instead of your 5.7....

chris