Not sure what the amp rating of my 3 wire alt is (55a?), probably low. Will it run a EFI system along with the wideband and fuel pump? Have a wideband and elec pump now, voltmeter shows 13 volts.

Problem would be at idle RPM if there is one. If you show 13.0 volts at idle and high RPM now you will PROBABLY be ok with the EFI. Unless you add some other high current item like a fan. The injectors would be the only new thing and they shouldn't be drawing much at low RPM.

You would probably be ok depending on fuel pump draw. The EFI computer itself is probably a 5-10A draw.

What definitively puts you over the top is any kind of electric cooling fan.

why not switch to a cs-120 off a latemodel cadillac they put out some real juice and cheap and easy to swap.

My old alternator is still working just fine. When I do the serp swap, then I'll put in a newer alt.

I wanted to stick with the 3-wire deal (voltage-sensing) rather than a single wire just b/c of what was stated here regarding the benefits. Any high-output 3 wire deals out there?

I don't know a PN, but I just got one off a later model truck I think. It is fine with my nitrous, 20A bottle heater, etc. Something meant for EFI and an electric fan should be good, unless you are going to run some monster fans, and other stuff.