I have a 440 in my 48 but it has been subframed. With stock frame rails I wouldnt think it would be to hard the only area I would think might get tight would be in the oil pump and filter area, steering can be taken care of with a borgeson joint or two, it may also help if you use a rear sump oil pan out of a truck. As for a small block,My buddy put a 360 in his coupe with stock front end, no clearance issues, just welded some mounts up and made a new trans crossmember for the 727, it was almost like it was made for that swap.
I hope you stick to your 383 idea there isnt many 46-48 Plymouth/Dodge street rods out there with Mopar power let alone BB Mopar