I've only worked on my projects so my experience is limited, but what I think I know is that torque determines getting going and hp determines top speed... Brian can confirm / shed more light...
Like Capt_orygun, the first thing to figure out is what % of your hp actually makes it to the driven wheels. My project is a bit more of an antique than yours, but in my case it's between 60% and 40% depending on transmission and AC and temperature and <long list>. My particular car had all options and an automatic transmission, so I'm at 40-45%. In the EV version most or all of those losses go away (the accessories still drain your battery, but they don't load the motor) so that helps.
Between eliminating that loss and the fact that you rarely rev the car to maintain cruising speed, I would think that a somewhat smaller hp electric motor should do fine. Like Brian said, the size of the motor is pretty much a moot point when it comes to efficiency/range, but trying to get high-hp motors can get expensive quick. Staying in the Warp-9/-11 range really helps the cost of a project stay "normal".
The one place where you do want "the power" is in getting off the line. That's when you rev an ICE to get the torque built up to accelerate, because an ICE has 0 torque at 0 rpm. The story is that an electric motor has full torque at 0 and that that makes it warp-speed / better than ICE. Well, it depends.
Like Brian says, you slip the clutch to let the engine rev up to get to torque. In my project, the ICE probably generates 250 ft lbs at 60% rpm, that's respectable (but the car weighs 6,000+ lbs so it needs it, it's no rocket). The E-motor equivalent generates 450 ft lbs, and that sounds great.
The Big Deal, the central decision here is whether to keep the original transmission.
My 1st gear is/was 4:1, so that 250 ft lbs from the ICE turns into 1,000 ft lbs at the wheel. Using the e-motor and no more transmission would now put me waaayy behind at 1/3 or half of what the original performance was, and that's before just flooring the ICE. The original 0-60 time was 10.6 seconds, and this would be more like 25 sec.
If you're keeping the transmission, then a lower-range electric motor will likely do fine. If you're ditching the transmission, you're giving up on all of the torque-multiplication so you'll have to buy a motor that can deliver them by itself. That makes a really big difference. A 250 ft lbs e-motor is "cheap", a 1,000 ft lbs e-motor is big and expensive.
In my project, because of the horrible hp-efficiency of the automatic transmission and because my e-motor is way too high in rpm for the transmission to handle anyway, I did ditch the transmission. To still get as much torque as I can, I've installed a permanent 2:1 gearbox (by TorqueBox). It lets the e-motor run at 6,000+ rpm so it's at peak efficiency, and it multiplies the torque to 900 ft lbs which is close enough to the original performance (or even better than in higher gears) to be "equal".
Anyway, my 2 cents