Best engine for caprice/impala clone

I am in the market for a 96/up caprice to convert to an impala clone. I will likely be getting a 4.3 v8 model as these are selling for @ $2100 dollars on ebay. Am going to want to pull this engine? I want impala style power and I cant see this little 4.3 pulling it n/a.
Next question is; as far as a new engine goes what should I be in the market for? LT1 or LS1? I have done a lot of internet searching and so far it seems that I should stick with the LT1 because the LS1 may not have the bottom end power to pull a 2 ton car with any kind of speed. Mid range and up? Sure, but not on the low end. Is this right?
On the LT1 what kind of mileage can I fuel expect? Can that be improved while maintaining power? I am told that the chevy 350 in the silverado can get up to 25 mpg with out losing power if tuned right. I have also heard that there is a performance chip company who builds an LS1 vette engine that gets 28 to 32 mpg without losing power. Realistic?
Any help at all will be great. Thanks.

You mentioned a 96/up Caprice but the 96 was the last year for the car so there is no "up."

If you want LT1 power, than why in the world would you spend money on a 4.3 Caprice? You should be able to locate an ex-police car well within your price range. The LT1's will last well over 100k miles and it should be less expensive doing the interior and exterior upgrades than doing an LT1 upgrade.

With a stock LT1, you should be able to get low to mid 20's on the road. From my experience, much above 75 and the mileage will drop off and that's with the stock 3.08's.