My Challenge to U /6ers....

I don't think it is possible within your $1500 budget, unless you do all the work yourself, and do not have to pay for machine shop work or welding.
I have a 64 valiant 170 slant six, three on the tree. Built for economy. Honest 9.5 to 1 engine, gets 27-29 mpg on the highway. Cost about $1200 to do the engine. To do what you want I would build the engine very similar, but lower compression to about 8.7 and use a low boost turbo setup. Limit boost to about 6-8 lbs. You must keep the compression up to have effecency off boost.