well, if you build a stroker you should be at around 400hp or more depending on heads, manifold, exhaust systems, and tons of torque. To me, the key here is the huge torque you can get out of a stroker. Torque, rather than just HP is what's key to a good street motor.
As an example, a crate ZZ383 stroker motor is usually around 425hp and 460 ft/lbs torque. I like that!
drop down about 15hp or so because they usually come with a single plane manifold and a dual plane would be better for a street motor - better low end torque and drivability.
As for pricing, your best bet would be to check at a good engine builder to see what they would charge you to build up a motor compared to the price of a crate motor. It's probably about the same or maybe a little less to purchase a crate motor anymore but I really don't know what the cost to have one custom built is currently.
one thing to keep in mind when dropping in a much higher performance motor is the weaknesses that you will find on the rest of the drivetrain. I don't know what componets are stock in a '75, but you will want to be sure your yokes, driveshaft, and halfshafts can handle the extra power - especially if you go to a wider and stickier rear tire. If these let go they tend to do so in a fairly "exciting" way and I don't think you would enjoy having a halfshaft come flying up through the floorpan at you while you are driving...........
