There is nothing budget about a 351 swap especially one that needs a rebuild.
while sticking with a 302 saves the specific 302-351 swap costs, if you include the cost of stroking the 302 to 347 (which is obviously not necessary with a 351), then (all other things being equal) you will may very vell find that the 351 swap actually costs less. it would be interesting to crunch the numbers on that.
as to budget mindedness, it depends on how one goes about it. doing what you are talking about (either with stroking a 302 or a stock 351 shortblock) new heads and cam and intake all at the same time ... sure, that is not going to be cheap either way.
but that is only one way to approach a build. another approach is take a phased approach: the first phase being to get the 351 in there AS IS (as much as possible) and then start saving for and planning the next phase of the build, that being the "big power" phase.
the positives of a phased approach (in my opinion) are:
(1) the initial cost is lower,
(2) the total cost is spread out over a longer period of time,
(3) the total project is broken into smaller, less expensive, shorter term sub-projects, with tangible progess that helps keep the motivation up (HUGE!)
(4) you might just find out you are happier that you thought you would be with the results of phase 1 (so phase 2 might become just a HCI upgrade as opposed to stroking it too), and
(5) you can actually enjoy the car after completing the first phase while saving up for and planning the next phase.
having lived through the "all at once" process, and seeing how long the car was out of comission and how much money i spent on it, i have no doubt at all that, had i taken a phased approach like i am talking about, i would have been happier every step of the way, and would have ended up with a different combination that cost me much less in the long run.