McLeod Racing - Bellhousings
520$ is pricey, could weld one up very nicely for that price, 2004r will do the job though. T5 would be a great swap, an s10 nv3500 would be a good fit for the cruiser too. NV3500 uses a wider gear spread than T5's, but overdrive is .73 vs .67 current or .68 T5. NV3500 would probably net better mpg, but only with a higher rear. The wider gear spread will help you get going quicker at extremely low rpms (less than 1000-1300rpm). NV3500 + 2.56 would be nuts. Would see high 20's regularly.
on the extreme side of things, I'd put money on 30mpg with a NV + 2.14 and flat ground if the 307's in good shape.