Wait... y'all are forgetting that the rotating assembly from an LT1 is the same as any 1-piece-rear SBC.
But... an LT1 is nothing special. The aluminum heads are basically a step above L98 heads. The LT1 iron heads are what they used to make Vortecs, so stock LT1s are basically upgraded TPIs or a Vortec SBC. No reason for the specialty intakes and higher prices to use the LT1 when it is functionally a duplicate of other stock SBCs.
Vortec heads (casting numbers end with 062 or 906) are super cheap stock castings that will do 400 hp easy. Valve lift should be kept under .470" unless you do one of the many aftermarket spring/retainer kits that allow .550".
The rest of the recipe is easy: Flat tops give about 9.5:1, good for up to about 224 degrees of intake duration. Set total timing to 32 degrees, initial to about 10-12, burn 87 octane all day and enjoy 375-400 hp. Small domes (don't mill Vortec heads much, they have thin decks) will give you 10:1. Pocket port the heads at home, up the cam a touch, and enjoy 450 hp or more on 91 octane.
Of course, headers and proper intake are necessary. Pick up a used Ebay Performer RPM for $100 and a Qjet for $20. Rebuild/tune it yourself and spend another $35 on a rebuild kit. Find an HEI from a junkyard for $25 and buy an $18 HEI recurve kit.
Its really simple. Just pick a recipe from this site and duplicate it: http://www.camaros.net/racing/SBCombo.html or here: http://www.chevelles.com/racing/SBCombo.html
There is another really good site with like 400+ combinations dyno tested, but I can't find it right now. EDIT: I found it... http://www.ryanscarpage.50megs.com/combos1.html
Concerning the 700r4, you won't hear me giving lots of love for them. They have a super deep 3.06 first and then a huge step to a 1.7x second. I used to autocross my Impala SS with the 4L60E (the electronic version of a 700r4) and it was a pain. Heavy throttle inputs leaving a turn it was reluctant to drop out of second so it bogged. Then when it did shift down to that 3.06 first, good luck not eating cones. In stock form they are pretty wimpy and have shift hunting issues... all of which are easily remedied, but not on challenge budgets. A good old fashioned TH350 is tried, true, better ratios, more reliable, and three gears is all you need for the challenge. You can buy a good used TH350 for $150 and add a shift kit yourself. Same basic strength/guts as the 700r4, but without the OD. Its also lighter :) Up to 1982 they were non-lockup TC. After 82 they were lockup. The lockups have some rare reliability issues where they don't unlock, but in a challenge situation where simplicity, cheapness, and only having what you need are key elements, I would stick with the older version. You won't need TC lockup anyway, so why add the teeny tiny risk of TC lockup failure and the potentially higher cost of the newer tranny? BUT, its what you have, so I suggest a shift kit and practice getting used to racing it with that big ratio change. That's cheaper than buying a whole different TH350 tranny and a longer driveshaft.
The 4.3 V6 is an SBC with two cylinders missing. All accessory drive bolt holes are the same, so any 4.3 or SBC belt drive will bolt on.