Cheap mods I have come across if you have a 67-70 mustang or similar chassis.
Here are a few cheap modifications I have come across that will either directly bolt on to the older cars or can be modified to work if you have a little fabrication skills.
The late 1990s Explorers seem to be a little bit of a treasure chest for both the new, and even the older cars.
The 8.8 rear end in the 1990s Explorers, many come with traction locks and 31 spline axles. Not to mention all the good gear ratios from 3.27 to 4.10s. What is interesting is the rear end is almost the same width (within 1/2 inch) as a 1967-1970 mustang. The axle tubes will need to be removed and swapped around so the center offset will be the same as the older car, and new leaf spring brackets welded on because the spacing is slightly different. But this is a relatively inexpensive way to gain rear wheel disc brakes, traction lock, and 31 spline axles. An entire assembly from a Pull A Part with traction lock, 3.73s, 31 spline axles, and disc brakes cost me $180. By the time you modified a 9" with all the aftermarket stuff you would have almost $2000 in it, and a less efficient rear end.
The aluminum driveshaft out of the 2 wheel drive Explorers is the correct length for a 1967-1970 mustang assuming you are using a small block and C4, toploader, or Tremec transmission. May even be the same with the FE powered mustangs that have a toploader as well but not sure. However, the Explorer driveshaft does use the wider truck style 1330 U-joints so you will need the correct yoke for the transmission and rear end. $30 driveshaft from a Pull A Part. Pretty cheap for an aluminum driveshaft.
Many people already know this, but the starter from the 5.0 Explorers is about half the weight and size compared to those older 1960s small block Ford starters. They clear headers on the older cars much better. Just make sure you know how to wire it up on the older cars otherwise the starter gear will stay engaged to the flywheel for a few seconds after startup. Doesn't take long to destroy a starter doing that. The same starter can also be found from any 1994-up 3.8 V6 mustang.
Many people already know this one also, but, the 302 in the late 1990s Explorer had the best flowing cylinder head of any of the high production Windsor engines. They also have lighter weight pistons than any of the older 302 engines. Would make an excellent transplant with a carbed intake for an older car that came with a 289 or 302. Or, if you wanted to go EFI on the older cars they had the best flowing mass produced upper and lower intake manifolds as well as throttle body. Last time I priced one from a Pull A Part they were $220.