I just dropped my t150 and sm420 bearing retainer off at the machine shop so an indexing ring can be made.

One thing I'm worried about is the part where the throwout bearing sits. its about 3 hundreths of an inch thicker on the t150 bearing retainer. will that difference cause a problem? the machine shop said making a sleeve would be really hard to do. Also, the t150 retainer is about .5" longer, will the length cause a problem?

If you are trying to use the 420 with a 150 bellhousing, yes you will have problems if the input shafts aren't the same length. The retainer length may or may not give troubles. I'm not sure I understand what you're doing.

thats what I'm trying to do, reuse the stock bellhousing. I just found out though the t150 won't work and I'll need a t5,t4,or sr4 bellhousing. not enough meat to drill new holes on the 150 bh.