[Christ]

The springs are a tuning issue that you'll have to figure out for yourself, they're similar to jetting in carbs.

As long as the fuel control gets it's signal from vacuum, it will not matter where you put the fuel input. You could (in theory) replace the injector ports with propane, if you were so inclined. Instead, you probably (since you mentioned piping the bottom of the intake) would want to drill and tap your injector fittings in at approx a 35* angle from the intake runner's floor. Any steeper than that, and you could be limiting fuel intake because of reversion. You want to keep the arc between the injector nozzle and the intake valve as low and long as possible so that fuel doesn't pool. (Or, in this case, create pockets.)

Ideally, any fuel jetted via injectors should actually be sprayed almost directly at the intake valve, but since propane is already vaporized, you should be OK moving the injector nozzles back about 4" from the intake ports and letting the propane and air mix on the way down the runner.