'87 witha 4.6l?

[84LX89GT]

The only problem with having a 4.6L installed in your vehicle is that you have to have ALL the emissions equipment hooked up. There
will be lots of rewiring and fabrication if you don't use an aftermarket fuel injection management system. A USED 4.6L DOHC can go anywhere from $2500-$3500 from what i've seen, then you can use a T-45 trans behind it. I just bought a '98 Cobra and i love it, but with 305 hp and 300 lb/ft of torque at high rpm's it really makes you appreciate the idea of regearing.
If i were you, i'd go with a 351W or a 351W stroker, real easy to get to 300 horsepower and loads more torque at lower rpm and easy to swap in, if you go carbureted it's very inexpensive (relatively).