Yes, I think so. You need a different bellhousing, but they are available with a SBC bolt pattern that will mate with the old 4-speeds. They're cast iron, and I've seen them all over E-bay for around the $50 range. I picked up one too.
As far as welding the subframe goes, you are probably forging the way. But if the width matches up well, it shouldn't be too hard. If not, then it becomes harder. But if you have all the equipment and are handy with a welder, it's probably do-able.
