First off, yes, replacing the expansion valve to a 134a piece when changing from R12 is a good move many overlook.
While the Redtek r12a may be better than 134a, I do doubt it is better than genuine R12. It is a blend using propane, and thus has some drawbacks - read on.
Using any blend replacement for R12, or 134a, has some serious repercussions. First off many of the blends are a propane blend - yup, the same stuff you use for your barbecue. As such, they are illegal for use in motor vehicles.
Second, blends are an AC garage's nightmare. They will contaminate their tanks. Most good AC shops now test all cars, especially old ones like ours, for signs of blend use. If they find it, be prepared to have them either refuse to service your system or charge you mucho bucks.
Next, many of the blends are sold as a DIY kit. They often include a can of sealer. Sealer in an AC system is much like sealer in a coolant system. Yup, it may very well fill the leak, but it then goes on to fill other small places you don't want filled. Again, sealer in an AC shop's equipment is the kiss of death to their units. BIG bucks to clean and repair. About the only time to use a sealer is a last ditch effort to keep things going for another season or two - knowing you are doing a full replacement upgrade when the money finally arrives.
An AC outfit in Arizona sponsors a very good online forum which is much like ours. It has a search feature, also. I've learned a lot from my time spent there:
http://tinyurl.com/ACkitsFORUM
Finally, I know it is common thought that moving the condenser up front is the thing to do. And it surely can not hurt. But we all know many Panteras with it still in the rear, even with fully stock systems, that manage to cool quite nicely.
Like me, JFFR says he is still playing with the system (welcome, fellow glutton for punishment-
) and thus his low system performance may or may not be due to the rear-mount condenser. He too is suffering with solving proper fill amounts for a modified system, with the added issue of using a blend.As I wrote in an earlier post....
letting a known, trusted shop figure this out for you would sure be a less-frustrating affair for you than the road I have taken.
Larry