I think you might be able to find that out on the www.hewland.com ('Gearbox ranges') site. I think they have a section for that has the Mk9 which I think is the later version. Also do a web search I think you can find them and info about them.
If memory serves me right the Mk 8 is a lightweight 4 speed version of the Mk5 usually run behind 1600cc Formula Ford engines etc. Same as a Mk9 but the Mk9 has output sideplates for mounting calipers for inboard discs. Definitely wouldn't work in any GT40 application.
The Mk6,8, and 9 are all low power gearboxes.I have one in my Lotus 35 F2/3 car. Its ok with about 150 or so bhp.
When Cosworth built 1600 F2 motors, ie FVA. Hewland introduced the FT200, (Formula Two 200bhp) Was first used in Lotus 47 the race version of Europa S1 or S2.
For a big V8 you really need to go up to a DG 4 speed.
Cost in UK about £5k for a reasonable one.
FT 200 about £4.5k,
M8 or 9 about £2k or £3.5 new.