i participated in a LS1 transplant into a 300ZX and it was ridiculously difficult. my friend was an idiot and tried to copy one of the entrants in the ultimate street car challenge of sport compact car.
here is a short list of things that you need:
new custom engine mounts
some custom front wheel drive transaxle that will mate a small block
custom drive shafts
or you go rear wheel drive and you need to cut a tunnel to fit the trans, then redesign the rear suspension to accept a rear axle
even if you get that far, 6.2L of chevy torque will tear the chassis apart. and with an engine that huge the v8 roar will kinda not make it that sleeper.
I wouldn't make it FWD... If anything IF you wanted to do it, convert it to RWD...
With that being said, depending on your shop's labor rate and all that they do to it... My estimate, labor alone, would be probably right around $15000... I know custom shop labor rates here are $65/hr or so. It's a big conversion with a BIG price tag.
Can't be any harder than a Chevy 327 sideways in the back seat of a 1960 Mini. Saw one in Milwaukee years ago.
This might be easier;
Find a Corvette that has been rolled.
Make a couple fiberglass molds of the xB ala funny car.
Fit the modified shell to the Corvette chassis.
Now you have something that will go & stop and sort of looks like an xB.