the 1985-1987 hf tranny will fit, the 1984-1987 dx and base model tranny will fit to and only give you slightly better gearing then you have, but not nearly as high as the HF but you should find out if your engine feels ok running at 1,500-2,000 rpm, just try it, shift in to 5th gear in town.
I would not mess with the vacuum lines other then replace them with new lines, removing them or the emissions equipment on these cars tends to kill mileage, performance and over heat the engine till it dies.
I have a passenger side mirror delete from my '84 civic hatch that I am willing to part with for $5 plus shipping if you can wait till the weather is above freezing.
No one ever changes the pcv valve (under the intake manifold on the other models), or tranny fluid (every other year!), wide tires are commen but a set of 13" rims with 155/80 R13 tires would be best for mileage.
If you need any parts, I have a '84 dx that was rear ended on the drivers side, still has alot of good part, time + shipping, I also have the PDF of the dealer shop manual at my web site
Right Here check it out.