I'd either go with a Herald/Vitesse chassis which is only 1/2" longer wheelbase than the 300 or, if you want to use a Scimitar SS1 chassis I'd change the styling to the 200si, which came with an 86.6" wheelbase -
The 200si is a prettier car imho and came with an inline 4 cylinder twin cam as standard, so it should be easier to get a suitable engine into the package - notice how the 200si bonnet is lower between the front wings than the 300 - making it look and sound more authentic. Here's a 200 and a 300 next to each other for comparison -
Nice selection of pictures in the link below -
http://www.maserati-alfieri.co.uk/alfieri279.htm