Yes assuming you could press off the Pistons and install to the replacement HSC Rods without damaging them, then it would certainly be a big improvement. My 1977 250 Maverick had even more than the earlier 250's (was about .180 down the bore with C.R. Of 8.0 to 1 or less). For an example I will do the math using the stock Ford specs, but you should measure yours when you pull the head. In many cases I have found that the actual measurements are usually considerably more (taller deck) than the stated minimum Ford specs. In talking with one of the Ford engineers FTF, this was done so that if a mistake was made or damage occurred from a bad cutter during its machining at the Ford engine plant there was still enough material left to clean it up without having to scrap the part.
The Ford spec block deck height of a stock 250 is 9.469", this measurement also won't account for I f there was any milling during an engines rebuild.
The Ford USA 250 4.1L Connecting Rod is 5.880" long
The Ford USA 2.5L HSC Taurus Connecting Rod is 5.990" long.
The Ford USA 200 / 250 Pistons have a (CH) Compression Height of 1.50 inches
The Ford Austrailan 200 / 250 Pistons have a Compression Height of 1.530 inches
So a 250 with stock rod and piston combo, the piston will end up about 0.134" down the bore.
250 Crankshaft has a 3.91"stroke, dived by / 2 = 1.955 + Rod length of 5.880" + a Piston CH of 1.50" = a combined total of 9.335 - the Block Deck Height of 9.469 gives us a piston .134 down the bore. Now with the tipical FelPro head gasket (excellent) of .050 thick plus the .134 down the bore we have Quench Distance of .184
When you substute the 5.990 2.5L rod (that's .110 longer) with your 250 piston, the piston will now only be 0.024" down the bore.This will help a lot with your quench, and increase the compression ratio, plus the stock 250 rod to stroke ratio changes from 1.504 to 1.532 also another slight benefit. Now we have a much better Quench Distance too, .050 plus .024 = .074. If you then were to use the Austrailan Spec Piston too with its 1.530 CH this would bring the Quence Distance smack dab in the middle (at .044) of the more ideal range for Quench distance (.035 to .050) which helps in the fight against detonation when you start raising the compression ratio of a US 250 engine. And if you wanted too you could also use the Victor head gasket at .045 thick to lose another .005 of Quench, good luck