This question comes up all the time..
If you said what you are prepared to spend?
Or in stages.. If you wanted to open engine etc...
Its been well proven this engine in GOOD condition will run all day on 400 h.p....
Keep in mind with even 350 to 400 rwh.p your torque is nearly doubled, or feels like it has. This makes things hard on transmissions. But makes car MUCH nicer to drive..
Stage one would be to leave engine totally std.. Run around 8Lb boost with 2 T3 turbo's.. A single GT35R ,T04Z, T66 to T70 sized single too..
Its the TUNE that makes engine safe.. Upgraded fuel pump, bigger injectors, colder spark plugs..I guess it would cost $5 to $7k at this level to build, depending how much you do yourself..
A stand alone ECU IMO is mandatory...Its major part of good tune and impossible without one as the are designed to read boost etc..
To go the next step in performance your looking at MUCH more money ..
Rebuilt or another engine built with forged low compression pistons and forged rods..
To do head mods can get expensive!! Changing the cam buckets at $22 each X's 32 ...It seems the cam std cam and exhaust side is the area that needs attention..
Steel Cometic or MLS heads gaskets and head stud conversion is best to help prevent head lifting on high boost levels..
Then the transmission won't handle to torque.. So you enter the unknown area and get std auto upgraded or convert to home grown auto ?Which is a known torque / power capacity.. As in Ford C4, AOD 4 speeds and G.M's T350 / 400 and there 4 speed equivalent auto's..
Converting to manual ? The home grown [well Mexico] TKO etc..
The Toyota Supra manual conversion is popular ...
On tune , what works etc
www.turbomustangs.com is a wealth of information.. What would work on pushrod 5.0 would work well on 4.0 cammer turbo size wise..
The early 1UZ's had better rods than latter model...