Drivetrain Questions and Suggestions

For $1800, up here buys very little, so My suggestion is;
Before you do anything, go out and do some TIMED zero to 60mph runs with the current combo.Do at least three and average the time out. You will need these later to compare to, as your combo gets progressively quicker. After she's been hot-rodded a while you will get used to it and begin to think she is slow.
So this year,spring of year#1
Put a new seal in the tranny, and it is is still shifting good,put it back in.
Then put some 4.10s, plus an LSD, into the 8.75. Sell the 8.25, and buy some big tires. Then call it done. Those 4.10s will get you about 60mph@4800 at the top of second gear.This will be all for this year.In the meantime you can save up.
Year#2,spring
Install headers, a free flowing exhaust and a 4bbl . Put those on there, all together,for another nice boost. These are all things the 360 will need anyway.
Now you are ready for more engine if it comes to that. I say if, cuz those listed items will make a huge improvement in the teeners performance.So start saving for the third spring.
Try not to be tempted to hotrod the teener. For about the same money plus a core, you can have a nice 5.9. This is an instant 13.5% increase in engine size,with an instant increase in across-the-board performance,even stock.
Year #3;
Drop that stock,used 5.9 in there, and freshen up the tranny.Drive it for the summer.
Year#4 saving money; and time to upgrade; the suspension,steering,&brakes, to prepare for the planned performance improvements.
Year #5;saving money; and upgrades to the cooling, ignition, and fueling systems,for the coming bigger engine.
Year #6. build a nice 5.9, and drop that 380hp SBMer in there,as a climax to the program.

Now you have something.