I only suggested the 41 because the 45 wouldn't fit.
But for my money, if it was mine i would research it for a while, a good week on the net, find out the biggest and best that would look stock but give it some grunt.
Remember pre 76 had no ADR's so the sky is really the limit.
A good set of headers, with a decent exhaust system, maybe a webber, like the 40mm twin down draught IDE's, about 9:1 comp, see someone about a cam maybe, any luminition type (optical) electronic ignition and you would have a very nice vehicle.
BUT! If you want an out & out BEAST, VQ41 would be my go.
Cheers, D