Yes it will fit. The 4.9 is physically identical to the 4.5 in the Allante. Have you bought the Allante yet or you're just thinking about it? If you do stick the 4.9 into the Allante, my recommendation would be to use the Allante heads on the 4.9. The Allante heads support another extra 1000 rpms over the 4.9 heads. Once it's all installed, you'll need to install an adjustable FPR to dial in the extra fuel requirements of the extra displacement engine. Anyways, this sounds like a very neat project. I also have a 88 Allante with a 4.1 and if I ever have to do engine work on it, I will convert it over to a 4.9. With the fancy Allante intake manifold, you'll have more power than a stock 4.9. I would say you would get an extra 20+ horsepower.