Vacuum is vacuum any where in the line, one shoudlnt affect the other. I would suggest an hydraulic power combo kit for your brakes, the cost is not too mush more considering all the parts that go with a vacuum kit.
http://67-72chevytrucks.com/vboard/s...threadid=65013
The majority of your engine weight is on the engine mounts. An automatic crossmember at the tail is all you need.
I am going with the crate 383 with HOT cam and possibly fast burn heads, 460-480 ft-lbs. But $$$$.