Plenty of people put 390's and 460's in 48-52 F-1's. They are a tight fit around the steering box but there are literally dozens of them on the Ford Truck Enthusiast site,
https://www.ford-trucks.com/forums/forum41/. Personally, I never liked either the FE's or the 385-series, an SBC drops in far more easily.
PS -- I can't believe what the PO did to the trans crossmember! That must be one flexible frame!