Read this first.
http://www.rotaryeng.net/simple-cheap-555.html

just had another idea.

Use a cheap CPU with a look up table who's only function
is to modify the voltage sent to the 555 as a function
of manifold pressure. This correction or "trim" in EFI parlance can
be edited in a spread sheet and downloaded using the USB.
Henry Nee's idea.

Atmel chips have a similar asm instruction set to a 6502 or 68K.
ZBasic sell's them on boards with USB software drivers.

http://www.zbasic.net/

The program would be so simple it would be easy and quick to do and maintain.

Main loop:
read the manifold pressure a to d
read the rpm.
Look up the correction.
Apply the correction to the d to a output pin
connected to the 555 pin 5, pulse width control pin.
Do it over again.

Paul