5.3/SM465 swap problems

I put a 99 5.3L in my 1973 chevy, Im running a SM465 4 speed and 208 T case. I used a 0411 ECM and harness from an 02 6.0L truck and a 02 5.7 manual camaro tune file. So theres my parts list. Ive been driving it like 6 months and it works ok but it doesnt seem like it has the power it should and in 3rd gear it falls flat out on its face at 4800 rpm. I have a dakota pulse generator for VSS input but I dont have a clutch or brake switch hooked up yet. Ive found it only reads the base spark table and I think thats my 4800 flat spot but I dont know how what I need to do to make it read the high/low spark tables. Also it seems to run like crap if you lug it down, just stutters. But it does run decent and it gets me where I gotta go, just needs a lot of refining. Sorry for the long post but any ideas would be great, it gets exhausting trying to solve it on your own. thanks