wots the cheapest way to go from peg-leg to LSD without welding it up???

slapping in an second hand lsd hemisphere and retaining your gears/shims and hoping for the best.

how much damage r we talking? less then $200 ?

I'd suggest start scanning through the for sale sections of various ford related forums & search eBay. You can sometimes find a good value disc to disc LSD diff for an E series falcon which makes the swap very easy. Perhaps even try the wreckers near you. If it is for the EA then getting one from an EA would be cheapest (& easiest). Different models used slightly different control arms, but they are easy enough to get. EA's used a different flange for the tailshaft, but that can be swapped too (or use the later model tailshaft if you have a 4 or 5 speed). Going to a later model diff will have the benefit of 28 spline axles though (EA & EB1 3.9L models had 25 spline).

Steer clear of minispools on eBay claiming to do the same job as an LSD though. They are illegal for street use, as is a welded centre (there is a sticky explaining different diff types). They claim to make it a full time LSD, but they actually lock the axles together, which is great on a dry race track (sensational actually, until you snap an axle) but impossible in the car park & they chew through control arm bushings quickly. But worse still they are very hairy & scary in the wet. The car wants to face the other way any time you move the throttle, on or off.

so i cant just get a centre? the diff was recently rebuilt so im not too keen on swapping it for something else unless i really have to

Yes, as Krytox posted you can just get the centre. Make sure it is the same spline & from the same housing though.

weld it up