Re: cheap 302 upgrades
Not sure on the exact cost the crank could be around $200 the money would be getting the crank, rods balanced etc, phone a machine shop, my guess around $500-$600 all up. And the motor would have to be pulled down so more expense in labour, so it might not be worth it, since your only 16 a 302 should be more than enough!! geez I could only drive my olds 4cyl when I was that age and p's when I was 18.
Change the diff ratio and get the extractors put on, don't play with the motor until it needs be pulled down, considering your 16 my guess would be 1year max, maybe 2 years!
What kind of diff? 8 3/4 b/w diff? You'll have to pull the centre out, not sure on those diffs never done one, but it can't be hard, get an XD V8 Gregorys manual, will have everything you need to know in it. Also put a thread in the diff/trans area.
I'd go 3.45 or even 3.25 will improve your acceleration heaps. Another suggestion I have would be to take the centre out and give it to a diff shop to do the ratio change and check everything out like the LSD clutches and bearing, again since your young the diff centre could be rooted, burnouts and clutch dumping kills diffs, you might needs a few parts replaced.
No offence coz your 16, I was that age once and know what goes on, I had a mate blow three diffs in a year!!! He had an XF 6cyl manual and really gave it stick.
Avoid hard dumping of the clutch, the shudder through the gearbox and diff kills them.
Avoid burnouts from dirt onto the road, when the tires bight the road also kills diff