They're both good engines. In 350 form, I believe the TPI, even mildly built up with better heads and cam, would be stronger in the low rpm and midrange than an LT1. But in 327 form, there is enough deficit in cubic inch-derived torque to make the LT1 probably about the same torque-wise. So the difference there would be more of a wash.
Where the LT1 shines is its capability to make power above the rpm of a TPI, and its reverse-cooling set up. The heads have a perfect heart-shaped combustion chamber design for excellent swirl, and the cooling system cools the heads first, and then the block. I would have to check, but I think the LT1 runs on pump gas with something like 11.5:1 compression, and I know it has a hydraulic roller cam. Peak RPM is about 6500, compared to 4400 to 5200 on the TPI, depending on how you cam it and the quality of the heads.
I have had an opportunity to compare a 350 TPI, nicely cammed, to a 350 LT1, first-hand. In a 350, my TPI made so much torque that even at a mile in altitude, you could burn rubber easily and "drift" for about a city block. It was much more powerful up to around 4500 rpm than the LT1 was. But then above 4500 rpm, the LT1 pulled a lot better and made up the difference, about when the TPI was flattening out.
I also had a 327 and a 350 in the same car, and the torque difference between those two engines is substantial. So I predict that the difference between the torque of a TPI 327 and an LT1 350 is negligible. Although I'm more of a TPI fan, myself, I think the LT1 is an unbeatable deal and you should jump on it. Then I might suggest you finish the 327 and then compare the two, see what you would like better. But I feel in this case the LT1 350 would be the stronger engine.
