Lags the issue with yours losing poke could have been the camshaft?.
The a12 I have is slightly higher comp and churns well at all speeds
and overtakes with ease 100kmh and over.

It actually runs out of first too quickly so I know with a 3.7 it should
be a little better. A 3.54 will be the beez neez and its no obsession
just prefer using the torque rather than hp around town not to mention
hating first gear and almost able to take off in 2nd comfortably.

I remember the a15 in my coupe with 79mm bores been a real stump puller
with mild cam but useless without a 5 speed & 1st might as well not exist.
The stock camshaft made it even worse with all torque.

Using the calc with 165/70/13 and 1:1 final at 100kmh the 3.9 is at 3700rpm
no wonder I wasnt happy with the noise!!! With 3.7 this drops to 3500rpm so
will need an overdrive if using an A12

a10 piston velocity at 7000rpm redline is 826.000 m/min 3500rpm is 413m/min
a12 '' '' '' '' 980.000 m/min '' '' 490m/min
a14 '' '' '' '' 1078.000 m/min '' '' 539m/min
a15 '' '' '' '' 1148.000 m/min '' '' 574m/min

So with 3.7 rear at 1:1 and A10 piston travels
77 metres less than A12,
126 metres less than a14
161 metres less than an A15
to cruise at 100km/h

All changes with a rare overdrive 5 speed.