The northstar is good(based on personal experience). BTW, it's being discontinued either this year or next yr---notice how cadillac doesn't use v-8's anymore? Except in the suv's which are LS'es.
But the small block will be much easier/cheaper/faster to get running. So, i personally, would go small block. Incidentally, recently i've been thinking about doing a lambo conversion---to try to make a profit on.
The other question is how much power can the TH-125 transaxle handle?