your step 1 with a 351 is very different if you are going 351 route.
You can grab a set of used pulleys.
You should go ahead and take care of the rear end and gears.
Your catback of choice.
what is the time frame for building the 351? If 6 months, only buy the parts for the 351. If 2 years, you might go ahead and put on aluminum HCI now that would be suited for the 351s flow capabilities.
You can buy trick flow heads for around 1000 (see them for as low as 750 used) and toss a systemax II and a Cam on there maybe Once you get everything squared around, you can also have your 351 rebuilt with the trick flow forged pistons down the road.
Time frame is probably one to two years. I will be building the 351 "in the background" while doing the suspension, rear end, paint, wheels, brakes, exhaust, etc. I could probably take the GT-40 heads (and intake if it has one) off this 351 and install on the 302 with a cam in the mean time. Are there other 302 upgrades that are suitable for the 351 as well?