Well, if I thought the shortblock was serviceable, and budget is a consideration (when isn't it) I would do what I said above.
As for specific parts? Probably a set of 440source heads (or better, pro maxx) milled down to 75 cc, and a steel shim head gasket,, in place of the 90cc iron stockers (pick up 1-1 1/2 points compression) a very mild hydraulic cam (still more aggressive than a two barrel stocker!) probably a small cam and lifter kit from summit house brand, or better, a small lunati, around 215-220 at .050. Then an edelbrock rpm (or one of the cheap copies) and either a 650 or so edelbrock carb or brawler. Finish off with an inexpensive set of headers to fit the chassis.
If I was forced to make a guess, I think that combo would pick up 100hp (or more) over a stock 400 2bbl, and still be a nice driver.
That is not what I am gonna do with my 400, however.