Jonnie
You can fit a 4.6 engine with your existing setup - you just need to swap over the front cover and camshaft for a long-nose type - and buy a 20 quid crank spacer from Turners or RPi
You are taking a massive risk tough in buying a 500 quid 4.6 from somewhere.
My recommendation is to remove you engine as previously planned, strip it down and send the block off to TVR power for top hat liners(believe it or not they are the cheapest at 550 plus VAT)
Then buy a full rebuild kit from Turners including a kent H180 camshaft rather than a standard -
This will consist of mains, shells, rings, seals, gaskets, new head bolts, timing gear, lifters etc
Your pushrods and rockers should be ok but you must renew everything else.
Budget 750 for this.
An engineering shop will charge you a couple of hundred to polish your crank and give a very light skim and valve re-seat to the heads - don't forget to do this.
so about 1500 quid will give you, in effect, a brand new engine.
If you are confident you can do this it is the best way to go - in any case there is loads of help and assistance to help you through it.
A 3.9 with H180 cam will knock the socks off a dodgy 140000-mile 4.6 any day of the week.
Best of luck with it mate