If you wanna keep the 700r4 and use a 205 t-case, you have to go to Advance Adapters and either pickup a the shorter output shaft for like $180 (but then you gotta pay buku $$$$ to get the thing installed in the 700) or you get the Advance Adapters actually adapter for between the 700r4 and 205 for like $460; either way kinda pricey!!!
I would be worried about the reliability of a 700r4 behind a BBC (esp. if its a stock 700). There is a reason the 700 never came from the factory behind a 454...the Th400 was reserved for that task...sure you can pay to get a built 700 like a Raptor or an Art Carr(like Harley mentioned)...Personally, if you wanna stick with an auto, get a Th400 and learn to live with high RPM on the highway, or pay to get a built 700, or get a 4L80-E and be done with it...A NV4500 would be a sweet choice IMO...row your own gears, improve driveline angles and it's a stout piece...The tranny is kinda pricy and so is hooking it up for what you want, but it would be SWEET...You could always put the 700 in now and start saving money and when you smoke the 700r4, get what you want...
Chris