I always like the Olds V8, 260 through 403 especially but actually like anything but the sbc or LS swaps. The easiest swap is a Buick 350, if you can find a good one. Basically a direct swap, including accessories. A short tail TH350 with the BOP pattern or TH2004R are good trans options. I assume you have the 3 speed TH200C, a time bomb behind more power. Good luck👍