Yeah that's the bad part..that you had it smallblock mocked..you could still go ls..it would be fine with the t56..I don't know how far along you got with your frame fab but if it was me I would remove all the mounts and redo them to fit a LS/t56 rather than adapt it to fit..so that would mean some more fab work..but it would still probably be cheaper than doing the smallblock the way you were headed..with the bonus of a more modern engine and some decent gas mileage..I know what you mean though. I looked for a cheap LS block for mockup and can't find one.. but mine will be LS and I'm heavily considering turbos..but under the hood turbos will be a tight fit so I'm thinking about mounting them in the bed..nebs has a good looking setup with his bed mounted turbo on about page 11 of his build.
http://67-72chevytrucks.com/vboard/s...d.php?t=380212