A series is definately best for budget unless you can modify everything your self to allow a bigger engine. To go a L series or CA you will need bigger tunnel, struts + diff/tailshaft. Then you will need to get it engineered to be legal and that is like $400?? unless you know someone. Plus i'm sure there is plenty more dollars to spend to get in a bigger than A series engine in.

For budget go the A series. Same struts, cross member, gearbox, diff, tail shaft etc. Spend what it would cost on all this stuff on the A series and youd have a pretty quick streeter.