My opinion, go with a 69 block 351. Have it bored out to 357 and install good forged internals. With a set of decent heads(like AFR 185's), 8.5-9.0:1 compression, and all the boost the S trim can put out you should be in 600rwhp range. You should be able to do that relatively cheap compared to a R302 or Dart block. I don't think the S trim will make much more power than that, the most I've seen from one is 650rwhp with the setup I listed above except with a Dart 347.

