I have an open 742 case 3.23 geared 8 3/4. Everything looks good on the unit. I want to make it a SG. I see I can buy SG units for anywhere between $300-$350. My question is what would it cost, assuming I bought the SG and brought the stuff somewhere to have them set it up for me. I don't want to use an "unknown" center and feel I don't need to spend $800-$1000 for a new one. Thanks.

you cannot turn a open center unit in to a suregrip. you have to install a SG unit into your case. usually a shop charges 200/250 for setup plus around 150.00 for a install kit not counting the cost of the SG unit...
for the best deal around here get ahold of Dr Diff here on Moparts ...

For what you would pay, and time you would spend to make up your own center section, you could buy a new remanufactured 742 case and be way ahead of the game. I bought my 742 case 4.30 SG from Dr. Diff for a super good deal, and ready to install

all you would need to do is install new bearings on the sg, swap the ring gear over from the open carrier to the sg carrier and put it into the housing and set backlash and check your pattern. id guess a decent shop would charge you less than 100 bucks to do everything if you provided the parts.