Early Chevy 350's still had the front motor mount holes drilled, I have a 350 in my '56 Tow Truck, it dropped right in using the stock front mounts and the side bellhousing mounts.
I don't think the GMC 302 6 cylinder bellhousing will bolt to a Chevy SB. If you can find a Chevy Truck V8 (not car) bellhousing your in business. You might need to get the Chevy V8 front mount brackets that are sold by most vendors.
Mike
