hey i have a 1987 150 conversion van that i "inherited" from my dad. it has a 5.0 EFI engine and a 4 speed overdrive auto transmission and it started its life as a bare van ( slider, no rear windows, brown work truck that my dad baught for his repair business) and when it was deemed kindof sucky for entering and exiting driveways it was sent to DMC for a converstion kit, after which the added weight made it even slower. to top it off when my family tried towing with it (given the trailor was probobly triple the rated weight, but proper mods other than power were given) it would struggle to move and break 55mph. we did the following power mods to it:
gibson 3" exhaust
Headers (i think gibson but i don't know for sure)
hyper chip (recently replaced after it started malfunctioning with a hypertech chip)
stronger ignition coil
some other ignition thing
K&N filter (which probobly didn't add power but it was a mod none the less)
the timing was also set slightly more aggressive and therefor needs 93octane fuel. but it doesn't run well on the 10% ethenol, luckily there are some stations using 100% gas
to my knowlege, all of these are still available, and after those it could tow that 6,000lb trailor with ease at 75mph no problem. i was also told (these mods were done 10 years ago probobly before i could drive) that it wouldn't hit 80 full speed after the conversion kit, and now easily soars past it. (although handling becomes an issue) before the power mods, with full conversion kit, both tanks filled, towing equipment (but no trailer) and the usual crap of a used vehicle inside (and extra stuff for the kids) it would go 0-60 in roughly 18-20 seconds, which totally sucks, and now it does it in 12 which is still slow but a huge improvement.
i can't tell (because all were done at once) which one is most responsible for the great power gain, but i have heard that the exhaust system is horribly restrictive and it responsible for much of it (and sounds really cool). also the engine would miss every now and then at low rpm under load (or up a hill) and after the hypertech chip was put it it runs perfect now.
as a side note, with 7 passengers it outran a 1993 econoline conversion van (my uncle's, also carrying 7passengers but i don't know which engine he has other than some V8)
if you are looking for engine mod's summitracing.com has tons of them, but i don't know about compatability. because atleast for my 5.0 windsor it just sends everything thta fits a ford small block to me so i don't know which fits and which doesn't, but research should fix that.
hope this long winded explination helps!
oh and for gas milage its about 16 mpg cruising 75 mph or so. don't know about city milage. i'd guess 10-12 | 
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