Got my motor in my 21 and my collector on the pass side (tt bassetts) all but touches my transom. Any sugestions on a set of exaust tips, maybe go 4" and try to line it up so the 4" tip goes over my 3.5" collector,  and then run a rubber hose that connects the 4" tip to the 4" part of my tt  bassetts?  

I know the ends of my TT bassetts are stepped, something like 4-1/2 down to 3-1/2 in two steps, can be cut down to suit needs so yes... 
if you like the look of the 4" exhaust tips, I see no reason not to cut them back and join it with the hose. Less backpressure if you have
a high HP motor and less header to burn yourself on  which I always found to be a good thing.

Rebel

I don't know the specs on your motor, but if your adding mufflers (baffles) to the insides then it would be a good trade off
besides, less back pressure on a motor that runs under load all the time will benifit from it. Specialy a big block 427.
Just remember when you drop backpressure (a good thing) you will need to ballance by increasing small amounts of fuel to
reduce the lean effect of lower back pressure, thus reducing the chance of burning valves.
next question is, how good are you at tunning your motor (carbs) 
only draw back I can see is your richen the mixture and increase power (torque) a little and burn more fuel.
Maybe someone else can chime in and shed more light on the subject than me on this 

Rebel

just a little bit more...
from reading a few articles on the net, plus talking to a couple friends that came in to work today (engine builders and racers)
the same answer kept coming up...  the issue is not back pressure as much as exhaust velocity.
I could go into much deatail as some of my friends did  but I'll let you read this and maybe draw your own conclusion.
http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm
sums up most of what I read and heard so far...
surprised no engine tunners have chimed in...  them that tinker should know lots on this subject.

Rebel

They are not effective enough to keep your noise levels down. For a stationary test, the noise may not exceed 88 decibels or 90 decibels
depending on the date of manufacture of the motor. So if your HP is too high, they are ineffective at reducing the levels.
Silencers that you insert into headers only reduce from 8 - 12 db. Here is a link to some info and mufflers that should work... 
http://shopping.rexmar.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=silencers-powersilencer
I think Tom at JBP could help you out with them mufflers if your worried about levels! 
http://www.jetboatperformance.com/index.php?p=bafflesandmufflers
Numbers there if you want to ask him questions...  he's a great help!

for me I just save my change in a jar (labeled "FUN TAX") just in case I get a ticket for those "fun" activities in my life  always turns out to be cheaper!

R3bel