I don't know JEM, but I do know Rover/TVR V8 engines...and £20k seems like a mahoosive cost for dropping in a simple FI V8 and R380 gearbox + a bit of fettling to the suspension and brakes. For example, I have a TVR Griffith 4.0 V8 with brand new R380 'box + new Hoyle Suspension/IRS and Brembo/Hi-Spec Motorsport brakes all round + 7.5 Ford Viscous LSD in my Sebring MGB. It cost me £15k in base components and another £2k in ancillaries and some 'trick' bits. It's a very high spec' 170mph set up.
You may like to talk with others who've completed a Landy V8 conversion...have a chat with John Eales for a relatively std 4.2 or 4.6 engine and Ashcroft Transmissions for a new HD R380 gearbox and there's myriad of suspension options [choose carefully]. Ultimately, this is just a Land Rover with a V8 conversion and there's nothing sophisticated/overly complex with this work. Can recommend you talk with Mark Adams at Tornado Systems when it comes to final rolling road set up and the conversion in general
https://www.tornadosystems.com/?fbclid=IwAR22ElQWsO92-qlXlAaTTE5layQbzMmsmU8xqclnAiXPy9IptVxR0amYSiw
and talk with the good guys on the V8 forum before you make final decision as to which engine builder you choose
https://www.v8forum.co.uk/forum/index.php