If you buy a complete 5.7 and run a carb and computer from XV, about the cheapest you can get all away with looks like around $5000 absolute best case scenario and HP will be around 400, the carb and headers 5.7 will make around 400 hp out the box.
For $5000 you can build quite a hot 340 stroker and make way more HP than 400. I am building my own 340 stroker for my 68 cuda and with home ported aluminum magnum heads, 1.7 mopar roller rockers, billet crower rods, wiseco pistons, forged mopar perf. crank, eddy arigap intake, comp XE268 cam TTI headers, rings, bearings, gaskets 770 street avenger or 850 T-quad I will have less than $5000 in it and I should be close to 500 hp and WAY more tq than a 5.7 could ever make with out spending big $$$ strokeing it, porting, caming... in a bucks down build the 5.7 stroker can run R E A L Y hard though! The ports flow as much OOTB as mine will ported, they have nice uniform thick bores and cross bolted mains. The 5.7 block is as good as some of the aftermarket small block blocks, the stock heads are better than SB eddys to boot
If you want EFI then the hemi will cost more also especially since your car is now wired for the SB injection.
If I was you I would build a stroker 5.2 or 5.9 magnum so I can use the roller cam and your existing injection set up.
You would have to get your computer reprogramed but the wiring can all stay.