The real basis for your swap might be this.
Figures like a 50 % or 1.5 factor loss for automatics based on the Turbo Hydramatic 3 speeds, but only 25% or approx 1.25 loss factor for a manual gearbox are common debits for flywheel DIN net hp to Rear wheel horsepower. As a passion killer for a road car, nought better than any kind GM Turbo Hydramatic, 3, 4 speed and its sucessor.
I'm lucky enough to have driven a 4 speed manual 5.3 V12 Jag. Great car, epic performance, but the old XK-E based Moss based gearbox was a heinous Sword in the Stone act of pot stiring when gearchanging, not a smooth sublime act compared to a Borg Warner 12 or THM 400 under kickdown. The original XJ-S V12 cars were fitted with either a manual or automatic transmission, but the manual and its optional in six cylinder Laycock overdrive 6 speed was soon dropped because it was a leftover from the XK-E and XJ sedan. The Rover 3.5 liter V8 and TR7 Triumph 77 mm 5 speed was its replacement. Same kind of box as a T5, or a Top Loader 3.03, only never ever anywhere near as strong as this two gearboxes can be.
From Sept 75 to 79, just 352 Moss box manual V12s were made out of 115,413 total from 1975 to 1996 or so.
In 1977, Jaguar fitted a different three-speed Turbo-Hydramatic 400 transmission from GM instead of Borg Warner. But the loss of performance with a 2.48:1 first 3.07 verses Jags 3.03" 4 speed manual was just like taking 48 hp off a 286 hp car. Early 3.07 axle 4 speed XJ-S had a 1st gear of 2.933:1, late ones might have been 3.238 with the 2.88 axle.
4speed, 3.07 axle, its 155 mph top speed, 14.9 second quarter mile , 6.5 sec 0-60 and 14 US mile per gallon highway 70 mph cruise consumption dropped to 136 mph, 16.6 seconds, 8.5 seconds and 11 mpg respectively with a THM 400.
Despite only 74 pound less weight in the gearbox, the cars weigh bridged at 265 pounds difference due to oil cooling system and appointment changes. The Moss box was pretty light, the THM 400 was well over 155 pounds.
See this
View: https://youtu.be/8NaQxnuEMAU